Introduce multiple clocks to GPR

52 files changed, 148 insertions, 101 deletions

diff --git a/include/grpc/support/time.h b/include/grpc/support/time.h
index 1fd3181859..fb8371bb46 100644
--- a/include/grpc/support/time.h
+++ b/include/grpc/support/time.h
@@ -62,8 +62,20 @@ extern const gpr_timespec gpr_inf_past;   /* The far past. */
 #define GPR_NS_PER_US 1000
 #define GPR_US_PER_MS 1000
 
-/* Return the current time measured from the system's default epoch. */
-gpr_timespec gpr_now(void);
+/* The clocks we support. */
+typedef enum {
+  /* Monotonic clock. Epoch undefined. Always moves forwards. */
+  GPR_CLOCK_MONOTONIC = 0,
+	/* Realtime clock. May jump forwards or backwards. Settable by
+	   the system administrator. Has its epoch at 0:00:00 UTC 1 Jan 1970. */
+  GPR_CLOCK_REALTIME
+} gpr_clock_type;
+
+/* initialize time subsystem */
+void gpr_time_init(void);
+
+/* Return the current time measured from the given clocks epoch. */
+gpr_timespec gpr_now(gpr_clock_type clock);
 
 /* Return -ve, 0, or +ve according to whether a < b, a == b, or a > b
    respectively.  */
diff --git a/src/core/channel/census_filter.c b/src/core/channel/census_filter.c
index 7e393a01a6..0aa3cc3710 100644
--- a/src/core/channel/census_filter.c
+++ b/src/core/channel/census_filter.c
@@ -149,7 +149,7 @@ static void client_init_call_elem(grpc_call_element* elem,
   call_data* d = elem->call_data;
   GPR_ASSERT(d != NULL);
   init_rpc_stats(&d->stats);
-  d->start_ts = gpr_now();
+  d->start_ts = gpr_now(GPR_CLOCK_REALTIME);
   d->op_id = census_tracing_start_op();
   if (initial_op) client_mutate_op(elem, initial_op);
 }
@@ -167,7 +167,7 @@ static void server_init_call_elem(grpc_call_element* elem,
   call_data* d = elem->call_data;
   GPR_ASSERT(d != NULL);
   init_rpc_stats(&d->stats);
-  d->start_ts = gpr_now();
+  d->start_ts = gpr_now(GPR_CLOCK_REALTIME);
   d->op_id = census_tracing_start_op();
   if (initial_op) server_mutate_op(elem, initial_op);
 }
@@ -176,7 +176,7 @@ static void server_destroy_call_elem(grpc_call_element* elem) {
   call_data* d = elem->call_data;
   GPR_ASSERT(d != NULL);
   d->stats.elapsed_time_ms =
-      gpr_timespec_to_micros(gpr_time_sub(gpr_now(), d->start_ts));
+      gpr_timespec_to_micros(gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), d->start_ts));
   census_record_rpc_server_stats(d->op_id, &d->stats);
   census_tracing_end_op(d->op_id);
 }
diff --git a/src/core/channel/client_setup.c b/src/core/channel/client_setup.c
index 5be8fa66e9..9844f4e43e 100644
--- a/src/core/channel/client_setup.c
+++ b/src/core/channel/client_setup.c
@@ -94,7 +94,7 @@ static void setup_initiate(grpc_transport_setup *sp) {
   int in_alarm = 0;
 
   r->setup = s;
-  r->deadline = gpr_time_add(gpr_now(), gpr_time_from_seconds(60));
+  r->deadline = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_seconds(60));
 
   gpr_mu_lock(&s->mu);
   GPR_ASSERT(s->refs > 0);
@@ -221,7 +221,7 @@ void grpc_client_setup_create_and_attach(
 int grpc_client_setup_request_should_continue(grpc_client_setup_request *r,
                                               const char *reason) {
   int result;
-  if (gpr_time_cmp(gpr_now(), r->deadline) > 0) {
+  if (gpr_time_cmp(gpr_now(GPR_CLOCK_REALTIME), r->deadline) > 0) {
     result = 0;
   } else {
     gpr_mu_lock(&r->setup->mu);
@@ -275,7 +275,7 @@ void grpc_client_setup_request_finish(grpc_client_setup_request *r,
     /* TODO(klempner): Replace these values with further consideration. 2x is
        probably too aggressive of a backoff. */
     gpr_timespec max_backoff = gpr_time_from_minutes(2);
-    gpr_timespec now = gpr_now();
+    gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
     gpr_timespec deadline = gpr_time_add(s->current_backoff_interval, now);
     GPR_ASSERT(!s->in_alarm);
     s->in_alarm = 1;
diff --git a/src/core/iomgr/iomgr.c b/src/core/iomgr/iomgr.c
index c47528aa94..39ba4f5264 100644
--- a/src/core/iomgr/iomgr.c
+++ b/src/core/iomgr/iomgr.c
@@ -59,7 +59,7 @@ static void background_callback_executor(void *ignored) {
   while (!g_shutdown) {
     gpr_timespec deadline = gpr_inf_future;
     gpr_timespec short_deadline =
-        gpr_time_add(gpr_now(), gpr_time_from_millis(100));
+        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_millis(100));
     if (g_cbs_head) {
       grpc_iomgr_closure *closure = g_cbs_head;
       g_cbs_head = closure->next;
@@ -67,7 +67,7 @@ static void background_callback_executor(void *ignored) {
       gpr_mu_unlock(&g_mu);
       closure->cb(closure->cb_arg, closure->success);
       gpr_mu_lock(&g_mu);
-    } else if (grpc_alarm_check(&g_mu, gpr_now(), &deadline)) {
+    } else if (grpc_alarm_check(&g_mu, gpr_now(GPR_CLOCK_REALTIME), &deadline)) {
     } else {
       gpr_mu_unlock(&g_mu);
       gpr_sleep_until(gpr_time_min(short_deadline, deadline));
@@ -89,7 +89,7 @@ void grpc_iomgr_init(void) {
   gpr_thd_id id;
   gpr_mu_init(&g_mu);
   gpr_cv_init(&g_rcv);
-  grpc_alarm_list_init(gpr_now());
+  grpc_alarm_list_init(gpr_now(GPR_CLOCK_REALTIME));
   g_root_object.next = g_root_object.prev = &g_root_object;
   g_root_object.name = "root";
   grpc_iomgr_platform_init();
@@ -110,7 +110,7 @@ void grpc_iomgr_shutdown(void) {
   grpc_iomgr_object *obj;
   grpc_iomgr_closure *closure;
   gpr_timespec shutdown_deadline =
-      gpr_time_add(gpr_now(), gpr_time_from_seconds(10));
+      gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_seconds(10));
 
   gpr_mu_lock(&g_mu);
   g_shutdown = 1;
@@ -145,9 +145,9 @@ void grpc_iomgr_shutdown(void) {
     if (g_root_object.next != &g_root_object) {
       int timeout = 0;
       gpr_timespec short_deadline =
-          gpr_time_add(gpr_now(), gpr_time_from_millis(100));
+          gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_millis(100));
       while (gpr_cv_wait(&g_rcv, &g_mu, short_deadline) && g_cbs_head == NULL) {
-        if (gpr_time_cmp(gpr_now(), shutdown_deadline) > 0) {
+        if (gpr_time_cmp(gpr_now(GPR_CLOCK_REALTIME), shutdown_deadline) > 0) {
           timeout = 1;
           break;
         }
diff --git a/src/core/iomgr/pollset_posix.c b/src/core/iomgr/pollset_posix.c
index 15ed8e75e6..852f2f4740 100644
--- a/src/core/iomgr/pollset_posix.c
+++ b/src/core/iomgr/pollset_posix.c
@@ -122,7 +122,7 @@ static void finish_shutdown(grpc_pollset *pollset) {
 
 int grpc_pollset_work(grpc_pollset *pollset, gpr_timespec deadline) {
   /* pollset->mu already held */
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   if (gpr_time_cmp(now, deadline) > 0) {
     return 0;
   }
diff --git a/src/core/iomgr/pollset_windows.c b/src/core/iomgr/pollset_windows.c
index 8d6bc79c96..c0573030e0 100644
--- a/src/core/iomgr/pollset_windows.c
+++ b/src/core/iomgr/pollset_windows.c
@@ -70,7 +70,7 @@ void grpc_pollset_destroy(grpc_pollset *pollset) {
 
 int grpc_pollset_work(grpc_pollset *pollset, gpr_timespec deadline) {
   gpr_timespec now;
-  now = gpr_now();
+  now = gpr_now(GPR_CLOCK_REALTIME);
   if (gpr_time_cmp(now, deadline) > 0) {
     return 0 /* GPR_FALSE */;
   }
diff --git a/src/core/iomgr/tcp_client_posix.c b/src/core/iomgr/tcp_client_posix.c
index bbf7711588..e0b0fd6e53 100644
--- a/src/core/iomgr/tcp_client_posix.c
+++ b/src/core/iomgr/tcp_client_posix.c
@@ -246,7 +246,7 @@ void grpc_tcp_client_connect(void (*cb)(void *arg, grpc_endpoint *ep),
   ac->write_closure.cb_arg = ac;
 
   gpr_mu_lock(&ac->mu);
-  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac, gpr_now());
+  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac, gpr_now(GPR_CLOCK_REALTIME));
   grpc_fd_notify_on_write(ac->fd, &ac->write_closure);
   gpr_mu_unlock(&ac->mu);
 
diff --git a/src/core/iomgr/tcp_client_windows.c b/src/core/iomgr/tcp_client_windows.c
index b1a169b519..2ae08f2435 100644
--- a/src/core/iomgr/tcp_client_windows.c
+++ b/src/core/iomgr/tcp_client_windows.c
@@ -215,7 +215,7 @@ void grpc_tcp_client_connect(void (*cb)(void *arg, grpc_endpoint *tcp),
   ac->refs = 2;
   ac->aborted = 0;
 
-  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac, gpr_now());
+  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac, gpr_now(GPR_CLOCK_REALTIME));
   socket->write_info.outstanding = 1;
   grpc_socket_notify_on_write(socket, on_connect, ac);
   return;
diff --git a/src/core/profiling/timers_preciseclock.h b/src/core/profiling/timers_preciseclock.h
index 163d52b797..5c251b47e6 100644
--- a/src/core/profiling/timers_preciseclock.h
+++ b/src/core/profiling/timers_preciseclock.h
@@ -82,7 +82,7 @@ struct grpc_precise_clock {
   gpr_timespec clock;
 };
 static void grpc_precise_clock_now(grpc_precise_clock* clk) {
-  clk->clock = gpr_now();
+  clk->clock = gpr_now(GPR_CLOCK_REALTIME);
 }
 #define GRPC_PRECISE_CLOCK_FORMAT "%ld.%09d"
 #define GRPC_PRECISE_CLOCK_PRINTF_ARGS(clk) \
diff --git a/src/core/security/credentials.c b/src/core/security/credentials.c
index cf663faf2d..4481554d74 100644
--- a/src/core/security/credentials.c
+++ b/src/core/security/credentials.c
@@ -384,7 +384,7 @@ static void jwt_get_request_metadata(grpc_credentials *creds,
     if (c->cached.service_url != NULL &&
         strcmp(c->cached.service_url, service_url) == 0 &&
         c->cached.jwt_md != NULL &&
-        (gpr_time_cmp(gpr_time_sub(c->cached.jwt_expiration, gpr_now()),
+        (gpr_time_cmp(gpr_time_sub(c->cached.jwt_expiration, gpr_now(GPR_CLOCK_REALTIME)),
                       refresh_threshold) > 0)) {
       jwt_md = grpc_credentials_md_store_ref(c->cached.jwt_md);
     }
@@ -401,7 +401,7 @@ static void jwt_get_request_metadata(grpc_credentials *creds,
       char *md_value;
       gpr_asprintf(&md_value, "Bearer %s", jwt);
       gpr_free(jwt);
-      c->cached.jwt_expiration = gpr_time_add(gpr_now(), c->jwt_lifetime);
+      c->cached.jwt_expiration = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), c->jwt_lifetime);
       c->cached.service_url = gpr_strdup(service_url);
       c->cached.jwt_md = grpc_credentials_md_store_create(1);
       grpc_credentials_md_store_add_cstrings(
@@ -586,7 +586,7 @@ static void on_oauth2_token_fetcher_http_response(
   status = grpc_oauth2_token_fetcher_credentials_parse_server_response(
       response, &c->access_token_md, &token_lifetime);
   if (status == GRPC_CREDENTIALS_OK) {
-    c->token_expiration = gpr_time_add(gpr_now(), token_lifetime);
+    c->token_expiration = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), token_lifetime);
     r->cb(r->user_data, c->access_token_md->entries,
           c->access_token_md->num_entries, status);
   } else {
@@ -608,7 +608,7 @@ static void oauth2_token_fetcher_get_request_metadata(
   {
     gpr_mu_lock(&c->mu);
     if (c->access_token_md != NULL &&
-        (gpr_time_cmp(gpr_time_sub(c->token_expiration, gpr_now()),
+        (gpr_time_cmp(gpr_time_sub(c->token_expiration, gpr_now(GPR_CLOCK_REALTIME)),
                       refresh_threshold) > 0)) {
       cached_access_token_md =
           grpc_credentials_md_store_ref(c->access_token_md);
@@ -623,7 +623,7 @@ static void oauth2_token_fetcher_get_request_metadata(
     c->fetch_func(
         grpc_credentials_metadata_request_create(creds, cb, user_data),
         &c->httpcli_context, pollset, on_oauth2_token_fetcher_http_response,
-        gpr_time_add(gpr_now(), refresh_threshold));
+        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), refresh_threshold));
   }
 }
 
diff --git a/src/core/security/google_default_credentials.c b/src/core/security/google_default_credentials.c
index 5822ce6337..93e609456e 100644
--- a/src/core/security/google_default_credentials.c
+++ b/src/core/security/google_default_credentials.c
@@ -105,7 +105,7 @@ static int is_stack_running_on_compute_engine(void) {
   grpc_httpcli_context_init(&context);
 
   grpc_httpcli_get(&context, &detector.pollset, &request,
-                   gpr_time_add(gpr_now(), max_detection_delay),
+                   gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), max_detection_delay),
                    on_compute_engine_detection_http_response, &detector);
 
   /* Block until we get the response. This is not ideal but this should only be
diff --git a/src/core/security/json_token.c b/src/core/security/json_token.c
index 6116f1d767..3c411d76cf 100644
--- a/src/core/security/json_token.c
+++ b/src/core/security/json_token.c
@@ -207,7 +207,7 @@ static char *encoded_jwt_claim(const grpc_auth_json_key *json_key,
   grpc_json *child = NULL;
   char *json_str = NULL;
   char *result = NULL;
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   gpr_timespec expiration = gpr_time_add(now, token_lifetime);
   char now_str[GPR_LTOA_MIN_BUFSIZE];
   char expiration_str[GPR_LTOA_MIN_BUFSIZE];
diff --git a/src/core/statistics/census_rpc_stats.c b/src/core/statistics/census_rpc_stats.c
index 0491c91947..3e571b1143 100644
--- a/src/core/statistics/census_rpc_stats.c
+++ b/src/core/statistics/census_rpc_stats.c
@@ -157,7 +157,7 @@ static void record_stats(census_ht* store, census_op_id op_id,
         key.ptr = gpr_strdup(key.ptr);
         census_ht_insert(store, key, (void*)window_stats);
       }
-      census_window_stats_add(window_stats, gpr_now(), stats);
+      census_window_stats_add(window_stats, gpr_now(GPR_CLOCK_REALTIME), stats);
     } else {
       census_internal_unlock_trace_store();
     }
@@ -185,7 +185,7 @@ static void get_stats(census_ht* store, census_aggregated_rpc_stats* data) {
   if (store != NULL) {
     size_t n;
     unsigned i, j;
-    gpr_timespec now = gpr_now();
+    gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
     census_ht_kv* kv = census_ht_get_all_elements(store, &n);
     if (kv != NULL) {
       data->num_entries = n;
diff --git a/src/core/statistics/census_tracing.c b/src/core/statistics/census_tracing.c
index 05e72b99c0..3b31a02cb3 100644
--- a/src/core/statistics/census_tracing.c
+++ b/src/core/statistics/census_tracing.c
@@ -94,7 +94,7 @@ census_op_id census_tracing_start_op(void) {
     g_id++;
     memcpy(&ret->id, &g_id, sizeof(census_op_id));
     ret->rpc_stats.cnt = 1;
-    ret->ts = gpr_now();
+    ret->ts = gpr_now(GPR_CLOCK_REALTIME);
     census_ht_insert(g_trace_store, op_id_as_key(&ret->id), (void*)ret);
     gpr_log(GPR_DEBUG, "Start tracing for id %lu", g_id);
     gpr_mu_unlock(&g_mu);
@@ -122,7 +122,7 @@ void census_tracing_print(census_op_id op_id, const char* anno_txt) {
   trace = census_ht_find(g_trace_store, op_id_as_key(&op_id));
   if (trace != NULL) {
     census_trace_annotation* anno = gpr_malloc(sizeof(census_trace_annotation));
-    anno->ts = gpr_now();
+    anno->ts = gpr_now(GPR_CLOCK_REALTIME);
     {
       char* d = anno->txt;
       const char* s = anno_txt;
@@ -144,7 +144,7 @@ void census_tracing_end_op(census_op_id op_id) {
   trace = census_ht_find(g_trace_store, op_id_as_key(&op_id));
   if (trace != NULL) {
     trace->rpc_stats.elapsed_time_ms =
-        gpr_timespec_to_micros(gpr_time_sub(gpr_now(), trace->ts));
+        gpr_timespec_to_micros(gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), trace->ts));
     gpr_log(GPR_DEBUG, "End tracing for id %lu, method %s, latency %f us",
             op_id_2_uint64(&op_id), trace->method,
             trace->rpc_stats.elapsed_time_ms);
diff --git a/src/core/statistics/window_stats.h b/src/core/statistics/window_stats.h
index d733d8d247..9651198c71 100644
--- a/src/core/statistics/window_stats.h
+++ b/src/core/statistics/window_stats.h
@@ -90,11 +90,11 @@
     // Record a new event, taking 15.3ms, transferring 1784 bytes.
     stat.latency = 0.153;
     stat.bytes = 1784;
-    census_window_stats_add(stats, gpr_now(), &stat);
+    census_window_stats_add(stats, gpr_now(GPR_CLOCK_REALTIME), &stat);
     // Get sums and print them out
     result[kMinInterval].statistic = &sums[kMinInterval];
     result[kHourInterval].statistic = &sums[kHourInterval];
-    census_window_stats_get_sums(stats, gpr_now(), result);
+    census_window_stats_get_sums(stats, gpr_now(GPR_CLOCK_REALTIME), result);
     printf("%d events/min, average time %gs, average bytes %g\n",
            result[kMinInterval].count,
            (my_stat*)result[kMinInterval].statistic->latency /
diff --git a/src/core/support/cancellable.c b/src/core/support/cancellable.c
index 5a4d488dd3..4f8f237f40 100644
--- a/src/core/support/cancellable.c
+++ b/src/core/support/cancellable.c
@@ -122,7 +122,7 @@ void gpr_cancellable_cancel(gpr_cancellable *c) {
           gpr_event ev;
           gpr_event_init(&ev);
           gpr_event_wait(&ev,
-                         gpr_time_add(gpr_now(), gpr_time_from_micros(1000)));
+                         gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000)));
         }
       }
     } while (failures != 0);
diff --git a/src/core/support/log_linux.c b/src/core/support/log_linux.c
index 48349d2c83..26f1feb5b3 100644
--- a/src/core/support/log_linux.c
+++ b/src/core/support/log_linux.c
@@ -73,7 +73,7 @@ void gpr_default_log(gpr_log_func_args *args) {
   char *final_slash;
   const char *display_file;
   char time_buffer[64];
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   struct tm tm;
 
   final_slash = strrchr(args->file, '/');
diff --git a/src/core/support/log_posix.c b/src/core/support/log_posix.c
index afca792c40..940ee20f15 100644
--- a/src/core/support/log_posix.c
+++ b/src/core/support/log_posix.c
@@ -75,7 +75,7 @@ void gpr_default_log(gpr_log_func_args *args) {
   char *final_slash;
   const char *display_file;
   char time_buffer[64];
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   struct tm tm;
 
   final_slash = strrchr(args->file, '/');
diff --git a/src/core/support/log_win32.c b/src/core/support/log_win32.c
index d249be7d2e..629781da8a 100644
--- a/src/core/support/log_win32.c
+++ b/src/core/support/log_win32.c
@@ -82,7 +82,7 @@ void gpr_log(const char *file, int line, gpr_log_severity severity,
 /* Simple starter implementation */
 void gpr_default_log(gpr_log_func_args *args) {
   char time_buffer[64];
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   struct tm tm;
 
   if (localtime_s(&tm, &now.tv_sec)) {
diff --git a/src/core/support/sync_win32.c b/src/core/support/sync_win32.c
index cc31d9b052..22387103fe 100644
--- a/src/core/support/sync_win32.c
+++ b/src/core/support/sync_win32.c
@@ -89,7 +89,7 @@ int gpr_cv_wait(gpr_cv *cv, gpr_mu *mu, gpr_timespec abs_deadline) {
   if (gpr_time_cmp(abs_deadline, gpr_inf_future) == 0) {
     SleepConditionVariableCS(cv, &mu->cs, INFINITE);
   } else {
-    gpr_timespec now = gpr_now();
+    gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
     gpr_int64 now_ms = now.tv_sec * 1000 + now.tv_nsec / 1000000;
     gpr_int64 deadline_ms =
         abs_deadline.tv_sec * 1000 + abs_deadline.tv_nsec / 1000000;
diff --git a/src/core/support/time_posix.c b/src/core/support/time_posix.c
index afb58ef231..d5797e0db2 100644
--- a/src/core/support/time_posix.c
+++ b/src/core/support/time_posix.c
@@ -55,22 +55,55 @@ static gpr_timespec gpr_from_timespec(struct timespec ts) {
   return rv;
 }
 
-gpr_timespec gpr_now(void) {
+/** maps gpr_clock_type --> clockid_t for clock_gettime */
+static clockid_t clockid_for_gpr_clock[] = {
+  CLOCK_MONOTONIC,
+  CLOCK_REALTIME
+};
+
+void gpr_time_init(void) {}
+
+gpr_timespec gpr_now(gpr_clock_type clock) {
   struct timespec now;
-  clock_gettime(CLOCK_REALTIME, &now);
+  clock_gettime(clockid_for_gpr_clock[clock], &now);
   return gpr_from_timespec(now);
 }
 #else
 /* For some reason Apple's OSes haven't implemented clock_gettime. */
 
 #include <sys/time.h>
+#include <mach/mach.h>
+#include <mach/mach_time.h>
+
+static double g_time_scale;
+static uint64_t g_time_start;
+
+void gpr_time_init(void) {
+  mach_timebase_info_data_t tb = { 0, 1 };
+  mach_timebase_info(&tb);
+  g_time_scale = tb.numer;
+  g_time_scale /= tb.denom;
+  g_time_start = mach_absolute_time();
+}
 
-gpr_timespec gpr_now(void) {
+gpr_timespec gpr_now(gpr_clock_type clock) {
   gpr_timespec now;
   struct timeval now_tv;
-  gettimeofday(&now_tv, NULL);
-  now.tv_sec = now_tv.tv_sec;
-  now.tv_nsec = now_tv.tv_usec * 1000;
+  double now_dbl;
+
+  switch (clock) {
+    case GPR_CLOCK_REALTIME:
+      gettimeofday(&now_tv, NULL);
+      now.tv_sec = now_tv.tv_sec;
+      now.tv_nsec = now_tv.tv_usec * 1000;
+      break;
+    case GPR_CLOCK_MONOTONIC:
+      now_dbl = (mach_absolute_time() - g_time_start) * g_time_scale;
+      now.tv_sec = now_dbl * 1e-9;
+      now.tv_nsec = now_dbl - now.tv_sec * 1e9;
+      break;
+  }
+
   return now;
 }
 #endif
@@ -83,7 +116,7 @@ void gpr_sleep_until(gpr_timespec until) {
   for (;;) {
     /* We could simplify by using clock_nanosleep instead, but it might be
      * slightly less portable. */
-    now = gpr_now();
+    now = gpr_now(GPR_CLOCK_REALTIME);
     if (gpr_time_cmp(until, now) <= 0) {
       return;
     }
diff --git a/src/core/support/time_win32.c b/src/core/support/time_win32.c
index f4443b5c2d..827abc741d 100644
--- a/src/core/support/time_win32.c
+++ b/src/core/support/time_win32.c
@@ -58,7 +58,7 @@ void gpr_sleep_until(gpr_timespec until) {
   for (;;) {
     /* We could simplify by using clock_nanosleep instead, but it might be
      * slightly less portable. */
-    now = gpr_now();
+    now = gpr_now(GPR_CLOCK_REALTIME);
     if (gpr_time_cmp(until, now) <= 0) {
       return;
     }
diff --git a/src/core/surface/call.c b/src/core/surface/call.c
index 181617fff8..82bd1b2de2 100644
--- a/src/core/surface/call.c
+++ b/src/core/surface/call.c
@@ -1190,7 +1190,7 @@ static void set_deadline_alarm(grpc_call *call, gpr_timespec deadline) {
   }
   GRPC_CALL_INTERNAL_REF(call, "alarm");
   call->have_alarm = 1;
-  grpc_alarm_init(&call->alarm, deadline, call_alarm, call, gpr_now());
+  grpc_alarm_init(&call->alarm, deadline, call_alarm, call, gpr_now(GPR_CLOCK_REALTIME));
 }
 
 /* we offset status by a small amount when storing it into transport metadata
diff --git a/src/core/surface/completion_queue.c b/src/core/surface/completion_queue.c
index 030a8b4e6f..3c074da652 100644
--- a/src/core/surface/completion_queue.c
+++ b/src/core/surface/completion_queue.c
@@ -325,7 +325,7 @@ void grpc_cq_hack_spin_pollset(grpc_completion_queue *cc) {
   gpr_mu_lock(GRPC_POLLSET_MU(&cc->pollset));
   grpc_pollset_kick(&cc->pollset);
   grpc_pollset_work(&cc->pollset,
-                    gpr_time_add(gpr_now(), gpr_time_from_millis(100)));
+                    gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_millis(100)));
   gpr_mu_unlock(GRPC_POLLSET_MU(&cc->pollset));
 }
 
diff --git a/src/core/surface/init.c b/src/core/surface/init.c
index ca61a38a35..db896934a3 100644
--- a/src/core/surface/init.c
+++ b/src/core/surface/init.c
@@ -33,6 +33,7 @@
 
 #include <grpc/census.h>
 #include <grpc/grpc.h>
+#include <grpc/support/time.h>
 #include "src/core/channel/channel_stack.h"
 #include "src/core/debug/trace.h"
 #include "src/core/iomgr/iomgr.h"
@@ -56,6 +57,7 @@ void grpc_init(void) {
 
   gpr_mu_lock(&g_init_mu);
   if (++g_initializations == 1) {
+    gpr_time_init();
     grpc_register_tracer("channel", &grpc_trace_channel);
     grpc_register_tracer("surface", &grpc_surface_trace);
     grpc_register_tracer("http", &grpc_http_trace);
diff --git a/src/core/transport/chttp2/parsing.c b/src/core/transport/chttp2/parsing.c
index 4664a0895c..a11b7e6d36 100644
--- a/src/core/transport/chttp2/parsing.c
+++ b/src/core/transport/chttp2/parsing.c
@@ -599,7 +599,7 @@ static void on_header(void *tp, grpc_mdelem *md) {
     }
     grpc_chttp2_incoming_metadata_buffer_set_deadline(
         &stream_parsing->incoming_metadata,
-        gpr_time_add(gpr_now(), *cached_timeout));
+        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), *cached_timeout));
     grpc_mdelem_unref(md);
   } else {
     grpc_chttp2_incoming_metadata_buffer_add(&stream_parsing->incoming_metadata,
diff --git a/src/core/transport/chttp2/stream_encoder.c b/src/core/transport/chttp2/stream_encoder.c
index cf78ac50cc..db642961ec 100644
--- a/src/core/transport/chttp2/stream_encoder.c
+++ b/src/core/transport/chttp2/stream_encoder.c
@@ -437,7 +437,7 @@ static void deadline_enc(grpc_chttp2_hpack_compressor *c, gpr_timespec deadline,
                          framer_state *st) {
   char timeout_str[GRPC_CHTTP2_TIMEOUT_ENCODE_MIN_BUFSIZE];
   grpc_mdelem *mdelem;
-  grpc_chttp2_encode_timeout(gpr_time_sub(deadline, gpr_now()), timeout_str);
+  grpc_chttp2_encode_timeout(gpr_time_sub(deadline, gpr_now(GPR_CLOCK_REALTIME)), timeout_str);
   mdelem = grpc_mdelem_from_metadata_strings(
       c->mdctx, grpc_mdstr_ref(c->timeout_key_str),
       grpc_mdstr_from_string(c->mdctx, timeout_str));
diff --git a/src/core/transport/metadata.c b/src/core/transport/metadata.c
index c80d67823f..4d0d83e7b7 100644
--- a/src/core/transport/metadata.c
+++ b/src/core/transport/metadata.c
@@ -161,7 +161,7 @@ grpc_mdctx *grpc_mdctx_create(void) {
   /* This seed is used to prevent remote connections from controlling hash table
    * collisions. It needs to be somewhat unpredictable to a remote connection.
    */
-  return grpc_mdctx_create_with_seed(gpr_now().tv_nsec);
+  return grpc_mdctx_create_with_seed(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
 }
 
 static void discard_metadata(grpc_mdctx *ctx) {
diff --git a/src/csharp/ext/grpc_csharp_ext.c b/src/csharp/ext/grpc_csharp_ext.c
index ec125db78b..a55cc9e247 100644
--- a/src/csharp/ext/grpc_csharp_ext.c
+++ b/src/csharp/ext/grpc_csharp_ext.c
@@ -379,7 +379,7 @@ grpcsharp_channel_args_destroy(grpc_channel_args *args) {
 
 /* Timespec */
 
-GPR_EXPORT gpr_timespec GPR_CALLTYPE gprsharp_now(void) { return gpr_now(); }
+GPR_EXPORT gpr_timespec GPR_CALLTYPE gprsharp_now(void) { return gpr_now(GPR_CLOCK_REALTIME); }
 
 GPR_EXPORT gpr_timespec GPR_CALLTYPE gprsharp_inf_future(void) {
   return gpr_inf_future;
diff --git a/src/php/ext/grpc/timeval.c b/src/php/ext/grpc/timeval.c
index 8a278d6760..ccf7f0f81a 100644
--- a/src/php/ext/grpc/timeval.c
+++ b/src/php/ext/grpc/timeval.c
@@ -208,7 +208,7 @@ PHP_METHOD(Timeval, similar) {
  * @return Timeval The current time
  */
 PHP_METHOD(Timeval, now) {
-  zval *now = grpc_php_wrap_timeval(gpr_now());
+  zval *now = grpc_php_wrap_timeval(gpr_now(GPR_CLOCK_REALTIME));
   RETURN_DESTROY_ZVAL(now);
 }
 
diff --git a/src/ruby/ext/grpc/rb_completion_queue.c b/src/ruby/ext/grpc/rb_completion_queue.c
index 8fb3949b3d..2d52d96dc8 100644
--- a/src/ruby/ext/grpc/rb_completion_queue.c
+++ b/src/ruby/ext/grpc/rb_completion_queue.c
@@ -91,7 +91,7 @@ static void grpc_rb_completion_queue_shutdown_drain(grpc_completion_queue *cq) {
    * - investigate further, this is probably another example of C-level cleanup
    * not working consistently in all cases.
    */
-  next_call.timeout = gpr_time_add(gpr_now(), gpr_time_from_micros(5e3));
+  next_call.timeout = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(5e3));
   do {
     rb_thread_call_without_gvl(grpc_rb_completion_queue_next_no_gil,
                                (void *)&next_call, NULL, NULL);
diff --git a/test/core/end2end/cq_verifier.c b/test/core/end2end/cq_verifier.c
index 33f7c02b61..c290cfdbad 100644
--- a/test/core/end2end/cq_verifier.c
+++ b/test/core/end2end/cq_verifier.c
@@ -248,7 +248,7 @@ void cq_verify(cq_verifier *v) {
 }
 
 void cq_verify_empty(cq_verifier *v) {
-  gpr_timespec deadline = gpr_time_add(gpr_now(), gpr_time_from_seconds(1));
+  gpr_timespec deadline = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_seconds(1));
   grpc_event ev;
 
   GPR_ASSERT(v->expect.next == &v->expect && "expectation queue must be empty");
diff --git a/test/core/fling/client.c b/test/core/fling/client.c
index ee5e390c39..6741a9dec8 100644
--- a/test/core/fling/client.c
+++ b/test/core/fling/client.c
@@ -124,7 +124,7 @@ static void step_ping_pong_stream(void) {
 }
 
 static double now(void) {
-  gpr_timespec tv = gpr_now();
+  gpr_timespec tv = gpr_now(GPR_CLOCK_REALTIME);
   return 1e9 * tv.tv_sec + tv.tv_nsec;
 }
 
diff --git a/test/core/fling/server.c b/test/core/fling/server.c
index 9542e15ad0..ac02407060 100644
--- a/test/core/fling/server.c
+++ b/test/core/fling/server.c
@@ -241,7 +241,7 @@ int main(int argc, char **argv) {
       shutdown_started = 1;
     }
     ev = grpc_completion_queue_next(
-        cq, gpr_time_add(gpr_now(), gpr_time_from_micros(1000000)));
+        cq, gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000000)));
     s = ev.tag;
     switch (ev.type) {
       case GRPC_OP_COMPLETE:
diff --git a/test/core/httpcli/httpcli_test.c b/test/core/httpcli/httpcli_test.c
index 6e579bc045..9e76a76666 100644
--- a/test/core/httpcli/httpcli_test.c
+++ b/test/core/httpcli/httpcli_test.c
@@ -145,7 +145,7 @@ int main(int argc, char **argv) {
   gpr_free(args[0]);
   gpr_free(args[2]);
 
-  gpr_sleep_until(gpr_time_add(gpr_now(), gpr_time_from_seconds(5)));
+  gpr_sleep_until(gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_seconds(5)));
 
   grpc_test_init(argc, argv);
   grpc_init();
diff --git a/test/core/iomgr/alarm_list_test.c b/test/core/iomgr/alarm_list_test.c
index 684e3f579a..461c268a23 100644
--- a/test/core/iomgr/alarm_list_test.c
+++ b/test/core/iomgr/alarm_list_test.c
@@ -51,7 +51,7 @@ static void cb(void *arg, int success) {
 }
 
 static void add_test(void) {
-  gpr_timespec start = gpr_now();
+  gpr_timespec start = gpr_now(GPR_CLOCK_REALTIME);
   int i;
   grpc_alarm alarms[20];
 
diff --git a/test/core/iomgr/alarm_test.c b/test/core/iomgr/alarm_test.c
index 0ccec435e6..362eb5fe63 100644
--- a/test/core/iomgr/alarm_test.c
+++ b/test/core/iomgr/alarm_test.c
@@ -113,7 +113,7 @@ static void test_grpc_alarm(void) {
   gpr_event_init(&arg.fcb_arg);
 
   grpc_alarm_init(&alarm, GRPC_TIMEOUT_MILLIS_TO_DEADLINE(100), alarm_cb, &arg,
-                  gpr_now());
+                  gpr_now(GPR_CLOCK_REALTIME));
 
   alarm_deadline = GRPC_TIMEOUT_SECONDS_TO_DEADLINE(1);
   gpr_mu_lock(&arg.mu);
@@ -165,7 +165,7 @@ static void test_grpc_alarm(void) {
   gpr_event_init(&arg2.fcb_arg);
 
   grpc_alarm_init(&alarm_to_cancel, GRPC_TIMEOUT_MILLIS_TO_DEADLINE(100),
-                  alarm_cb, &arg2, gpr_now());
+                  alarm_cb, &arg2, gpr_now(GPR_CLOCK_REALTIME));
   grpc_alarm_cancel(&alarm_to_cancel);
 
   alarm_deadline = GRPC_TIMEOUT_SECONDS_TO_DEADLINE(1);
diff --git a/test/core/iomgr/endpoint_tests.c b/test/core/iomgr/endpoint_tests.c
index 8198c24752..0cfba5fac8 100644
--- a/test/core/iomgr/endpoint_tests.c
+++ b/test/core/iomgr/endpoint_tests.c
@@ -254,7 +254,7 @@ static void read_and_write_test(grpc_endpoint_test_config config,
 
   gpr_mu_lock(GRPC_POLLSET_MU(g_pollset));
   while (!state.read_done || !state.write_done) {
-    GPR_ASSERT(gpr_time_cmp(gpr_now(), deadline) < 0);
+    GPR_ASSERT(gpr_time_cmp(gpr_now(GPR_CLOCK_REALTIME), deadline) < 0);
     grpc_pollset_work(g_pollset, deadline);
   }
   gpr_mu_unlock(GRPC_POLLSET_MU(g_pollset));
@@ -350,14 +350,14 @@ static void shutdown_during_write_test(grpc_endpoint_test_config config,
         deadline = GRPC_TIMEOUT_SECONDS_TO_DEADLINE(10);
         gpr_mu_lock(GRPC_POLLSET_MU(g_pollset));
         while (!write_st.done) {
-          GPR_ASSERT(gpr_time_cmp(gpr_now(), deadline) < 0);
+          GPR_ASSERT(gpr_time_cmp(gpr_now(GPR_CLOCK_REALTIME), deadline) < 0);
           grpc_pollset_work(g_pollset, deadline);
         }
         gpr_mu_unlock(GRPC_POLLSET_MU(g_pollset));
         grpc_endpoint_destroy(write_st.ep);
         gpr_mu_lock(GRPC_POLLSET_MU(g_pollset));
         while (!read_st.done) {
-          GPR_ASSERT(gpr_time_cmp(gpr_now(), deadline) < 0);
+          GPR_ASSERT(gpr_time_cmp(gpr_now(GPR_CLOCK_REALTIME), deadline) < 0);
           grpc_pollset_work(g_pollset, deadline);
         }
         gpr_mu_unlock(GRPC_POLLSET_MU(g_pollset));
diff --git a/test/core/iomgr/tcp_client_posix_test.c b/test/core/iomgr/tcp_client_posix_test.c
index b673c032b2..38f043339c 100644
--- a/test/core/iomgr/tcp_client_posix_test.c
+++ b/test/core/iomgr/tcp_client_posix_test.c
@@ -187,11 +187,11 @@ void test_times_out(void) {
   /* Make sure the event doesn't trigger early */
   gpr_mu_lock(GRPC_POLLSET_MU(&g_pollset));
   while (gpr_time_cmp(gpr_time_add(connect_deadline, gpr_time_from_seconds(2)),
-                      gpr_now()) > 0) {
-    int is_after_deadline = gpr_time_cmp(connect_deadline, gpr_now()) <= 0;
+                      gpr_now(GPR_CLOCK_REALTIME)) > 0) {
+    int is_after_deadline = gpr_time_cmp(connect_deadline, gpr_now(GPR_CLOCK_REALTIME)) <= 0;
     if (is_after_deadline &&
         gpr_time_cmp(gpr_time_add(connect_deadline, gpr_time_from_seconds(1)),
-                     gpr_now()) > 0) {
+                     gpr_now(GPR_CLOCK_REALTIME)) > 0) {
       /* allow some slack before insisting that things be done */
     } else {
       GPR_ASSERT(g_connections_complete ==
diff --git a/test/core/iomgr/tcp_server_posix_test.c b/test/core/iomgr/tcp_server_posix_test.c
index fb262711c0..110a2eb1d9 100644
--- a/test/core/iomgr/tcp_server_posix_test.c
+++ b/test/core/iomgr/tcp_server_posix_test.c
@@ -135,7 +135,7 @@ static void test_connect(int n) {
 
     gpr_log(GPR_DEBUG, "wait");
     while (g_nconnects == nconnects_before &&
-           gpr_time_cmp(deadline, gpr_now()) > 0) {
+           gpr_time_cmp(deadline, gpr_now(GPR_CLOCK_REALTIME)) > 0) {
       grpc_pollset_work(&g_pollset, deadline);
     }
     gpr_log(GPR_DEBUG, "wait done");
diff --git a/test/core/network_benchmarks/low_level_ping_pong.c b/test/core/network_benchmarks/low_level_ping_pong.c
index 78a0eef1a2..0ce4bd4b25 100644
--- a/test/core/network_benchmarks/low_level_ping_pong.c
+++ b/test/core/network_benchmarks/low_level_ping_pong.c
@@ -296,7 +296,7 @@ static void print_histogram(gpr_histogram *histogram) {
 }
 
 static double now(void) {
-  gpr_timespec tv = gpr_now();
+  gpr_timespec tv = gpr_now(GPR_CLOCK_REALTIME);
   return 1e9 * tv.tv_sec + tv.tv_nsec;
 }
 
diff --git a/test/core/statistics/census_log_tests.c b/test/core/statistics/census_log_tests.c
index 241ec1ce4f..a34dcf07c4 100644
--- a/test/core/statistics/census_log_tests.c
+++ b/test/core/statistics/census_log_tests.c
@@ -568,7 +568,7 @@ void test_performance(void) {
     double write_time_micro = 0.0;
     int nrecords = 0;
     setup_test(0);
-    start_time = gpr_now();
+    start_time = gpr_now(GPR_CLOCK_REALTIME);
     while (1) {
       void* record = census_log_start_write(write_size);
       if (record == NULL) {
@@ -577,7 +577,7 @@ void test_performance(void) {
       census_log_end_write(record, write_size);
       nrecords++;
     }
-    write_time = gpr_time_sub(gpr_now(), start_time);
+    write_time = gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), start_time);
     write_time_micro = write_time.tv_sec * 1000000 + write_time.tv_nsec / 1000;
     census_log_shutdown();
     printf(
diff --git a/test/core/statistics/multiple_writers_circular_buffer_test.c b/test/core/statistics/multiple_writers_circular_buffer_test.c
index a645e15918..56ada893ef 100644
--- a/test/core/statistics/multiple_writers_circular_buffer_test.c
+++ b/test/core/statistics/multiple_writers_circular_buffer_test.c
@@ -40,7 +40,7 @@
 
 int main(int argc, char **argv) {
   grpc_test_init(argc, argv);
-  srand(gpr_now().tv_nsec);
+  srand(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
   test_multiple_writers_circular_log();
   return 0;
 }
diff --git a/test/core/statistics/multiple_writers_test.c b/test/core/statistics/multiple_writers_test.c
index 84aef15c1a..e524927da6 100644
--- a/test/core/statistics/multiple_writers_test.c
+++ b/test/core/statistics/multiple_writers_test.c
@@ -40,7 +40,7 @@
 
 int main(int argc, char **argv) {
   grpc_test_init(argc, argv);
-  srand(gpr_now().tv_nsec);
+  srand(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
   test_multiple_writers();
   return 0;
 }
diff --git a/test/core/statistics/performance_test.c b/test/core/statistics/performance_test.c
index 3c1e28241e..3f0e080093 100644
--- a/test/core/statistics/performance_test.c
+++ b/test/core/statistics/performance_test.c
@@ -40,7 +40,7 @@
 
 int main(int argc, char **argv) {
   grpc_test_init(argc, argv);
-  srand(gpr_now().tv_nsec);
+  srand(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
   test_performance();
   return 0;
 }
diff --git a/test/core/statistics/quick_test.c b/test/core/statistics/quick_test.c
index 0e432314bb..c72ae77b98 100644
--- a/test/core/statistics/quick_test.c
+++ b/test/core/statistics/quick_test.c
@@ -40,7 +40,7 @@
 
 int main(int argc, char **argv) {
   grpc_test_init(argc, argv);
-  srand(gpr_now().tv_nsec);
+  srand(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
   test_invalid_record_size();
   test_end_write_with_different_size();
   test_read_pending_record();
diff --git a/test/core/statistics/small_log_test.c b/test/core/statistics/small_log_test.c
index c151b77f63..b26b95f639 100644
--- a/test/core/statistics/small_log_test.c
+++ b/test/core/statistics/small_log_test.c
@@ -40,7 +40,7 @@
 
 int main(int argc, char **argv) {
   grpc_test_init(argc, argv);
-  srand(gpr_now().tv_nsec);
+  srand(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
   test_small_log();
   return 0;
 }
diff --git a/test/core/statistics/window_stats_test.c b/test/core/statistics/window_stats_test.c
index d893f7f792..1c66a87407 100644
--- a/test/core/statistics/window_stats_test.c
+++ b/test/core/statistics/window_stats_test.c
@@ -83,7 +83,7 @@ void empty_test(void) {
   result.statistic = &sum;
   census_window_stats_get_sums(stats, zero, &result);
   GPR_ASSERT(result.count == 0 && sum.value1 == 0 && sum.value2 == 0);
-  census_window_stats_get_sums(stats, gpr_now(), &result);
+  census_window_stats_get_sums(stats, gpr_now(GPR_CLOCK_REALTIME), &result);
   GPR_ASSERT(result.count == 0 && sum.value1 == 0 && sum.value2 == 0);
   census_window_stats_destroy(stats);
 }
@@ -268,7 +268,7 @@ void rolling_time_test(void) {
   struct census_window_stats* stats =
       census_window_stats_create(1, &kMinInterval, 7, &kMyStatInfo);
   GPR_ASSERT(stats != NULL);
-  srand(gpr_now().tv_nsec);
+  srand(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
   for (i = 0; i < 100000; i++) {
     increment.tv_nsec = rand() % 100000000; /* up to 1/10th second */
     when = gpr_time_add(when, increment);
@@ -292,7 +292,7 @@ void infinite_interval_test(void) {
   gpr_timespec increment = {0, 0};
   struct census_window_stats* stats =
       census_window_stats_create(1, &gpr_inf_future, 10, &kMyStatInfo);
-  srand(gpr_now().tv_nsec);
+  srand(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
   for (i = 0; i < count; i++) {
     increment.tv_sec = rand() % 21600; /* 6 hours */
     when = gpr_time_add(when, increment);
diff --git a/test/core/support/cancellable_test.c b/test/core/support/cancellable_test.c
index b2db1afc76..e5ed77c7ca 100644
--- a/test/core/support/cancellable_test.c
+++ b/test/core/support/cancellable_test.c
@@ -81,22 +81,22 @@ static void test(void) {
   GPR_ASSERT(!gpr_cancellable_is_cancelled(&t.cancel));
 
   /* Test timeout on event wait for uncancelled gpr_cancellable */
-  interval = gpr_now();
+  interval = gpr_now(GPR_CLOCK_REALTIME);
   gpr_event_cancellable_wait(
-      &t.ev, gpr_time_add(gpr_now(), gpr_time_from_micros(1000000)), &t.cancel);
-  interval = gpr_time_sub(gpr_now(), interval);
+      &t.ev, gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000000)), &t.cancel);
+  interval = gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), interval);
   GPR_ASSERT(gpr_time_cmp(interval, gpr_time_from_micros(500000)) >= 0);
   GPR_ASSERT(gpr_time_cmp(gpr_time_from_micros(2000000), interval) >= 0);
 
   /* Test timeout on cv wait for uncancelled gpr_cancellable */
   gpr_mu_lock(&t.mu);
-  interval = gpr_now();
+  interval = gpr_now(GPR_CLOCK_REALTIME);
   while (!gpr_cv_cancellable_wait(
              &t.cv, &t.mu,
-             gpr_time_add(gpr_now(), gpr_time_from_micros(1000000)),
+             gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000000)),
              &t.cancel)) {
   }
-  interval = gpr_time_sub(gpr_now(), interval);
+  interval = gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), interval);
   GPR_ASSERT(gpr_time_cmp(interval, gpr_time_from_micros(500000)) >= 0);
   GPR_ASSERT(gpr_time_cmp(gpr_time_from_micros(2000000), interval) >= 0);
   gpr_mu_unlock(&t.mu);
@@ -113,7 +113,7 @@ static void test(void) {
   /* Wait a second, and check that no threads have finished waiting. */
   gpr_mu_lock(&t.mu);
   gpr_cv_wait(&t.cv, &t.mu,
-              gpr_time_add(gpr_now(), gpr_time_from_micros(1000000)));
+              gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000000)));
   GPR_ASSERT(t.n == n);
   gpr_mu_unlock(&t.mu);
 
@@ -129,21 +129,21 @@ static void test(void) {
 
   /* Test timeout on cv wait for cancelled gpr_cancellable */
   gpr_mu_lock(&t.mu);
-  interval = gpr_now();
+  interval = gpr_now(GPR_CLOCK_REALTIME);
   while (!gpr_cv_cancellable_wait(
              &t.cv, &t.mu,
-             gpr_time_add(gpr_now(), gpr_time_from_micros(1000000)),
+             gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000000)),
              &t.cancel)) {
   }
-  interval = gpr_time_sub(gpr_now(), interval);
+  interval = gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), interval);
   GPR_ASSERT(gpr_time_cmp(gpr_time_from_micros(100000), interval) >= 0);
   gpr_mu_unlock(&t.mu);
 
   /* Test timeout on event wait for cancelled gpr_cancellable */
-  interval = gpr_now();
+  interval = gpr_now(GPR_CLOCK_REALTIME);
   gpr_event_cancellable_wait(
-      &t.ev, gpr_time_add(gpr_now(), gpr_time_from_micros(1000000)), &t.cancel);
-  interval = gpr_time_sub(gpr_now(), interval);
+      &t.ev, gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000000)), &t.cancel);
+  interval = gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), interval);
   GPR_ASSERT(gpr_time_cmp(gpr_time_from_micros(100000), interval) >= 0);
 
   gpr_mu_destroy(&t.mu);
diff --git a/test/core/support/sync_test.c b/test/core/support/sync_test.c
index 44bc6ba471..bf5829c415 100644
--- a/test/core/support/sync_test.c
+++ b/test/core/support/sync_test.c
@@ -242,12 +242,12 @@ static void test(const char *name, void (*body)(void *m),
                  void (*extra)(void *m), int timeout_s) {
   gpr_int64 iterations = 1024;
   struct test *m;
-  gpr_timespec start = gpr_now();
+  gpr_timespec start = gpr_now(GPR_CLOCK_REALTIME);
   gpr_timespec time_taken;
   gpr_timespec deadline =
       gpr_time_add(start, gpr_time_from_micros(timeout_s * 1000000));
   fprintf(stderr, "%s:", name);
-  while (gpr_time_cmp(gpr_now(), deadline) < 0) {
+  while (gpr_time_cmp(gpr_now(GPR_CLOCK_REALTIME), deadline) < 0) {
     iterations <<= 1;
     fprintf(stderr, " %ld", (long)iterations);
     m = test_new(10, iterations);
@@ -265,7 +265,7 @@ static void test(const char *name, void (*body)(void *m),
     }
     test_destroy(m);
   }
-  time_taken = gpr_time_sub(gpr_now(), start);
+  time_taken = gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), start);
   fprintf(stderr, " done %ld.%09d s\n", (long)time_taken.tv_sec,
           (int)time_taken.tv_nsec);
 }
@@ -323,7 +323,7 @@ static void inc_with_1ms_delay(void *v /*=m*/) {
   for (i = 0; i != m->iterations; i++) {
     gpr_timespec deadline;
     gpr_mu_lock(&m->mu);
-    deadline = gpr_time_add(gpr_now(), gpr_time_from_micros(1000));
+    deadline = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000));
     while (!gpr_cv_wait(&m->cv, &m->mu, deadline)) {
     }
     m->counter++;
@@ -339,7 +339,7 @@ static void inc_with_1ms_delay_event(void *v /*=m*/) {
   gpr_int64 i;
   for (i = 0; i != m->iterations; i++) {
     gpr_timespec deadline;
-    deadline = gpr_time_add(gpr_now(), gpr_time_from_micros(1000));
+    deadline = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000));
     GPR_ASSERT(gpr_event_wait(&m->event, deadline) == NULL);
     gpr_mu_lock(&m->mu);
     m->counter++;
@@ -384,7 +384,7 @@ static void consumer(void *v /*=m*/) {
   gpr_mu_unlock(&m->mu);
   GPR_ASSERT(
       !queue_remove(&m->q, &value,
-                    gpr_time_add(gpr_now(), gpr_time_from_micros(1000000))));
+                    gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000000))));
   mark_thread_done(m);
 }
 
diff --git a/test/core/surface/completion_queue_test.c b/test/core/surface/completion_queue_test.c
index eba24f5c6e..66136cfe5e 100644
--- a/test/core/surface/completion_queue_test.c
+++ b/test/core/surface/completion_queue_test.c
@@ -69,7 +69,7 @@ static void test_wait_empty(void) {
   LOG_TEST("test_wait_empty");
 
   cc = grpc_completion_queue_create();
-  GPR_ASSERT(grpc_completion_queue_next(cc, gpr_now()).type ==
+  GPR_ASSERT(grpc_completion_queue_next(cc, gpr_now(GPR_CLOCK_REALTIME)).type ==
              GRPC_QUEUE_TIMEOUT);
   shutdown_and_destroy(cc);
 }
diff --git a/test/core/util/test_config.h b/test/core/util/test_config.h
index 0b3c54373d..3db13f5739 100644
--- a/test/core/util/test_config.h
+++ b/test/core/util/test_config.h
@@ -52,11 +52,11 @@ extern "C" {
   (GRPC_TEST_SLOWDOWN_BUILD_FACTOR * GRPC_TEST_SLOWDOWN_MACHINE_FACTOR)
 
 #define GRPC_TIMEOUT_SECONDS_TO_DEADLINE(x) \
-  gpr_time_add(gpr_now(),                   \
+  gpr_time_add(gpr_now(GPR_CLOCK_REALTIME),                   \
                gpr_time_from_micros(GRPC_TEST_SLOWDOWN_FACTOR * 1e6 * (x)))
 
 #define GRPC_TIMEOUT_MILLIS_TO_DEADLINE(x) \
-  gpr_time_add(gpr_now(),                  \
+  gpr_time_add(gpr_now(GPR_CLOCK_REALTIME),                  \
                gpr_time_from_micros(GRPC_TEST_SLOWDOWN_FACTOR * 1e3 * (x)))
 
 #ifndef GRPC_TEST_CUSTOM_PICK_PORT