1 files changed, 32 insertions, 29 deletions

diff --git a/absl/time/internal/cctz/src/time_zone_libc.cc b/absl/time/internal/cctz/src/time_zone_libc.cc
index 887dd097..af9f063e 100644
--- a/absl/time/internal/cctz/src/time_zone_libc.cc
+++ b/absl/time/internal/cctz/src/time_zone_libc.cc
@@ -125,31 +125,30 @@ inline std::tm* local_time(const std::time_t* timep, std::tm* result) {
 #endif
 }
 
-// Converts a civil second and "dst" flag into a time_t and UTC offset.
+// Converts a civil second and "dst" flag into a time_t and a struct tm.
 // Returns false if time_t cannot represent the requested civil second.
 // Caller must have already checked that cs.year() will fit into a tm_year.
-bool make_time(const civil_second& cs, int is_dst, std::time_t* t, int* off) {
-  std::tm tm;
-  tm.tm_year = static_cast<int>(cs.year() - year_t{1900});
-  tm.tm_mon = cs.month() - 1;
-  tm.tm_mday = cs.day();
-  tm.tm_hour = cs.hour();
-  tm.tm_min = cs.minute();
-  tm.tm_sec = cs.second();
-  tm.tm_isdst = is_dst;
-  *t = std::mktime(&tm);
+bool make_time(const civil_second& cs, int is_dst, std::time_t* t,
+               std::tm* tm) {
+  tm->tm_year = static_cast<int>(cs.year() - year_t{1900});
+  tm->tm_mon = cs.month() - 1;
+  tm->tm_mday = cs.day();
+  tm->tm_hour = cs.hour();
+  tm->tm_min = cs.minute();
+  tm->tm_sec = cs.second();
+  tm->tm_isdst = is_dst;
+  *t = std::mktime(tm);
   if (*t == std::time_t{-1}) {
     std::tm tm2;
     const std::tm* tmp = local_time(t, &tm2);
-    if (tmp == nullptr || tmp->tm_year != tm.tm_year ||
-        tmp->tm_mon != tm.tm_mon || tmp->tm_mday != tm.tm_mday ||
-        tmp->tm_hour != tm.tm_hour || tmp->tm_min != tm.tm_min ||
-        tmp->tm_sec != tm.tm_sec) {
+    if (tmp == nullptr || tmp->tm_year != tm->tm_year ||
+        tmp->tm_mon != tm->tm_mon || tmp->tm_mday != tm->tm_mday ||
+        tmp->tm_hour != tm->tm_hour || tmp->tm_min != tm->tm_min ||
+        tmp->tm_sec != tm->tm_sec) {
       // A true error (not just one second before the epoch).
       return false;
     }
   }
-  *off = static_cast<int>(tm_gmtoff(tm));
   return true;
 }
 
@@ -254,33 +253,37 @@ time_zone::civil_lookup TimeZoneLibC::MakeTime(const civil_second& cs) const {
   // We probe with "is_dst" values of 0 and 1 to try to distinguish unique
   // civil seconds from skipped or repeated ones.  This is not always possible
   // however, as the "dst" flag does not change over some offset transitions.
-  // We are also subject to the vagaries of mktime() implementations.
+  // We are also subject to the vagaries of mktime() implementations. For
+  // example, some implementations treat "tm_isdst" as a demand (useless),
+  // and some as a disambiguator (useful).
   std::time_t t0, t1;
-  int offset0, offset1;
-  if (make_time(cs, 0, &t0, &offset0) && make_time(cs, 1, &t1, &offset1)) {
-    if (t0 == t1) {
+  std::tm tm0, tm1;
+  if (make_time(cs, 0, &t0, &tm0) && make_time(cs, 1, &t1, &tm1)) {
+    if (tm0.tm_isdst == tm1.tm_isdst) {
       // The civil time was singular (pre == trans == post).
-      const time_point<seconds> tp = FromUnixSeconds(t0);
+      const time_point<seconds> tp = FromUnixSeconds(tm0.tm_isdst ? t1 : t0);
       return {time_zone::civil_lookup::UNIQUE, tp, tp, tp};
     }
 
-    if (t0 > t1) {
+    int offset = tm_gmtoff(tm0);
+    if (t0 < t1) {  // negative DST
       std::swap(t0, t1);
-      std::swap(offset0, offset1);
+      offset = tm_gmtoff(tm1);
     }
-    const std::time_t tt = find_trans(t0, t1, offset1);
+
+    const std::time_t tt = find_trans(t1, t0, offset);
     const time_point<seconds> trans = FromUnixSeconds(tt);
 
-    if (offset0 < offset1) {
+    if (tm0.tm_isdst) {
       // The civil time did not exist (pre >= trans > post).
-      const time_point<seconds> pre = FromUnixSeconds(t1);
-      const time_point<seconds> post = FromUnixSeconds(t0);
+      const time_point<seconds> pre = FromUnixSeconds(t0);
+      const time_point<seconds> post = FromUnixSeconds(t1);
       return {time_zone::civil_lookup::SKIPPED, pre, trans, post};
     }
 
     // The civil time was ambiguous (pre < trans <= post).
-    const time_point<seconds> pre = FromUnixSeconds(t0);
-    const time_point<seconds> post = FromUnixSeconds(t1);
+    const time_point<seconds> pre = FromUnixSeconds(t1);
+    const time_point<seconds> post = FromUnixSeconds(t0);
     return {time_zone::civil_lookup::REPEATED, pre, trans, post};
   }