Export of internal Abseil changes.

--
aa9e2bff92652605b8244677058be787c872f99c by Abseil Team <absl-team@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 202702969

--
d26c857c203589892a84bc44d789f2a15a60f234 by Abseil Team <absl-team@google.com>:

Cleans up the FixedArray code (formatting, renames, etc) without changing the functionality

PiperOrigin-RevId: 202538159
GitOrigin-RevId: aa9e2bff92652605b8244677058be787c872f99c
Change-Id: I6561257232c6cc8e1cbf51d7e26bae5f8760551e

15 files changed, 334 insertions, 139 deletions

diff --git a/absl/time/internal/cctz/include/cctz/time_zone.h b/absl/time/internal/cctz/include/cctz/time_zone.h
index 55804ba6..0b9764ea 100644
--- a/absl/time/internal/cctz/include/cctz/time_zone.h
+++ b/absl/time/internal/cctz/include/cctz/time_zone.h
@@ -119,7 +119,7 @@ class time_zone {
   // of the given civil-time argument, and the pre, trans, and post
   // members will give the absolute time answers using the pre-transition
   // offset, the transition point itself, and the post-transition offset,
-  // respectively (all three times are equal if kind == UNIQUE).  If any
+  // respectively (all three times are equal if kind == UNIQUE). If any
   // of these three absolute times is outside the representable range of a
   // time_point<seconds> the field is set to its maximum/minimum value.
   //
@@ -159,17 +159,79 @@ class time_zone {
   };
   civil_lookup lookup(const civil_second& cs) const;
 
+  // Finds the time of the next/previous offset change in this time zone.
+  //
+  // By definition, next_transition(tp, &trans) returns false when tp has
+  // its maximum value, and prev_transition(tp, &trans) returns false
+  // when tp has its minimum value. If the zone has no transitions, the
+  // result will also be false no matter what the argument.
+  //
+  // Otherwise, when tp has its minimum value, next_transition(tp, &trans)
+  // returns true and sets trans to the first recorded transition. Chains
+  // of calls to next_transition()/prev_transition() will eventually return
+  // false, but it is unspecified exactly when next_transition(tp, &trans)
+  // jumps to false, or what time is set by prev_transition(tp, &trans) for
+  // a very distant tp.
+  //
+  // Note: Enumeration of time-zone transitions is for informational purposes
+  // only. Modern time-related code should not care about when offset changes
+  // occur.
+  //
+  // Example:
+  //   cctz::time_zone nyc;
+  //   if (!cctz::load_time_zone("America/New_York", &nyc)) { ... }
+  //   const auto now = std::chrono::system_clock::now();
+  //   auto tp = cctz::time_point<cctz::seconds>::min();
+  //   cctz::time_zone::civil_transition trans;
+  //   while (tp <= now && nyc.next_transition(tp, &trans)) {
+  //     // transition: trans.from -> trans.to
+  //     tp = nyc.lookup(trans.to).trans;
+  //   }
+  struct civil_transition {
+    civil_second from;  // the civil time we jump from
+    civil_second to;    // the civil time we jump to
+  };
+  bool next_transition(const time_point<seconds>& tp,
+                       civil_transition* trans) const;
+  template <typename D>
+  bool next_transition(const time_point<D>& tp,
+                       civil_transition* trans) const {
+    return next_transition(detail::split_seconds(tp).first, trans);
+  }
+  bool prev_transition(const time_point<seconds>& tp,
+                       civil_transition* trans) const;
+  template <typename D>
+  bool prev_transition(const time_point<D>& tp,
+                       civil_transition* trans) const {
+    return prev_transition(detail::split_seconds(tp).first, trans);
+  }
+
+  // version() and description() provide additional information about the
+  // time zone. The content of each of the returned strings is unspecified,
+  // however, when the IANA Time Zone Database is the underlying data source
+  // the version() std::string will be in the familar form (e.g, "2018e") or
+  // empty when unavailable.
+  //
+  // Note: These functions are for informational or testing purposes only.
+  std::string version() const;  // empty when unknown
+  std::string description() const;
+
+  // Relational operators.
+  friend bool operator==(time_zone lhs, time_zone rhs) {
+    return &lhs.effective_impl() == &rhs.effective_impl();
+  }
+  friend bool operator!=(time_zone lhs, time_zone rhs) {
+    return !(lhs == rhs);
+  }
+
   class Impl;
 
  private:
   explicit time_zone(const Impl* impl) : impl_(impl) {}
+  const Impl& effective_impl() const;  // handles implicit UTC
   const Impl* impl_;
 };
 
-// Relational operators.
-bool operator==(time_zone lhs, time_zone rhs);
-inline bool operator!=(time_zone lhs, time_zone rhs) { return !(lhs == rhs); }
-
 // Loads the named time zone. May perform I/O on the initial load.
 // If the name is invalid, or some other kind of error occurs, returns
 // false and "*tz" is set to the UTC time zone.
@@ -184,6 +246,7 @@ time_zone utc_time_zone();
 time_zone fixed_time_zone(const seconds& offset);
 
 // Returns a time zone representing the local time zone. Falls back to UTC.
+// Note: local_time_zone.name() may only be something like "localtime".
 time_zone local_time_zone();
 
 // Returns the civil time (cctz::civil_second) within the given time zone at
@@ -227,7 +290,7 @@ bool parse(const std::string&, const std::string&, const time_zone&,
 //   - %E*f - Fractional seconds with full precision (a literal '*')
 //   - %E4Y - Four-character years (-999 ... -001, 0000, 0001 ... 9999)
 //
-// Note that %E0S behaves like %S, and %E0f produces no characters.  In
+// Note that %E0S behaves like %S, and %E0f produces no characters. In
 // contrast %E*f always produces at least one digit, which may be '0'.
 //
 // Note that %Y produces as many characters as it takes to fully render the
@@ -254,7 +317,7 @@ inline std::string format(const std::string& fmt, const time_point<D>& tp,
 // Parses an input std::string according to the provided format std::string and
 // returns the corresponding time_point. Uses strftime()-like formatting
 // options, with the same extensions as cctz::format(), but with the
-// exceptions that %E#S is interpreted as %E*S, and %E#f as %E*f.  %Ez
+// exceptions that %E#S is interpreted as %E*S, and %E#f as %E*f. %Ez
 // and %E*z also accept the same inputs.
 //
 // %Y consumes as many numeric characters as it can, so the matching data
diff --git a/absl/time/internal/cctz/include/cctz/zone_info_source.h b/absl/time/internal/cctz/include/cctz/zone_info_source.h
index 4d9d8f87..20a76979 100644
--- a/absl/time/internal/cctz/include/cctz/zone_info_source.h
+++ b/absl/time/internal/cctz/include/cctz/zone_info_source.h
@@ -31,6 +31,11 @@ class ZoneInfoSource {
 
   virtual std::size_t Read(void* ptr, std::size_t size) = 0;  // like fread()
   virtual int Skip(std::size_t offset) = 0;  // like fseek()
+
+  // Until the zoneinfo data supports versioning information, we provide
+  // a way for a ZoneInfoSource to indicate it out-of-band.  The default
+  // implementation returns an empty std::string.
+  virtual std::string Version() const;
 };
 
 }  // namespace cctz
diff --git a/absl/time/internal/cctz/src/cctz_benchmark.cc b/absl/time/internal/cctz/src/cctz_benchmark.cc
index f13cb4ee..c97df78c 100644
--- a/absl/time/internal/cctz/src/cctz_benchmark.cc
+++ b/absl/time/internal/cctz/src/cctz_benchmark.cc
@@ -754,23 +754,21 @@ void BM_Zone_LoadAllTimeZonesCached(benchmark::State& state) {
 }
 BENCHMARK(BM_Zone_LoadAllTimeZonesCached);
 
-void BM_Zone_TimeZoneImplGetImplicit(benchmark::State& state) {
+void BM_Zone_TimeZoneEqualityImplicit(benchmark::State& state) {
   cctz::time_zone tz;  // implicit UTC
-  cctz::time_zone::Impl::get(tz);
   while (state.KeepRunning()) {
-    cctz::time_zone::Impl::get(tz);
+    benchmark::DoNotOptimize(tz == tz);
   }
 }
-BENCHMARK(BM_Zone_TimeZoneImplGetImplicit);
+BENCHMARK(BM_Zone_TimeZoneEqualityImplicit);
 
-void BM_Zone_TimeZoneImplGetExplicit(benchmark::State& state) {
+void BM_Zone_TimeZoneEqualityExplicit(benchmark::State& state) {
   cctz::time_zone tz = cctz::utc_time_zone();  // explicit UTC
-  cctz::time_zone::Impl::get(tz);
   while (state.KeepRunning()) {
-    cctz::time_zone::Impl::get(tz);
+    benchmark::DoNotOptimize(tz == tz);
   }
 }
-BENCHMARK(BM_Zone_TimeZoneImplGetExplicit);
+BENCHMARK(BM_Zone_TimeZoneEqualityExplicit);
 
 void BM_Zone_UTCTimeZone(benchmark::State& state) {
   cctz::time_zone tz;
diff --git a/absl/time/internal/cctz/src/time_zone_format.cc b/absl/time/internal/cctz/src/time_zone_format.cc
index 592ab7d3..1b023848 100644
--- a/absl/time/internal/cctz/src/time_zone_format.cc
+++ b/absl/time/internal/cctz/src/time_zone_format.cc
@@ -141,6 +141,9 @@ char* Format02d(char* ep, int v) {
 
 // Formats a UTC offset, like +00:00.
 char* FormatOffset(char* ep, int offset, const char* mode) {
+  // TODO: Follow the RFC3339 "Unknown Local Offset Convention" and
+  // generate a "negative zero" when we're formatting a zero offset
+  // as the result of a failed load_time_zone().
   char sign = '+';
   if (offset < 0) {
     offset = -offset;  // bounded by 24h so no overflow
diff --git a/absl/time/internal/cctz/src/time_zone_format_test.cc b/absl/time/internal/cctz/src/time_zone_format_test.cc
index 33c23984..a90dda76 100644
--- a/absl/time/internal/cctz/src/time_zone_format_test.cc
+++ b/absl/time/internal/cctz/src/time_zone_format_test.cc
@@ -64,6 +64,17 @@ void TestFormatSpecifier(time_point<D> tp, time_zone tz, const std::string& fmt,
   EXPECT_EQ("xxx " + ans + " yyy", format("xxx " + fmt + " yyy", tp, tz));
 }
 
+// These tests sometimes run on platforms that have zoneinfo data so old
+// that the transition we are attempting to check does not exist, most
+// notably Android emulators.  Fortunately, AndroidZoneInfoSource supports
+// time_zone::version() so, in cases where we've learned that it matters,
+// we can make the check conditionally.
+int VersionCmp(time_zone tz, const std::string& target) {
+  std::string version = tz.version();
+  if (version.empty() && !target.empty()) return 1;  // unknown > known
+  return version.compare(target);
+}
+
 }  // namespace
 
 //
@@ -453,8 +464,8 @@ TEST(Format, ExtendedSecondOffset) {
   EXPECT_TRUE(load_time_zone("America/New_York", &tz));
   tp = convert(civil_second(1883, 11, 18, 16, 59, 59), utc);
   if (tz.lookup(tp).offset == -5 * 60 * 60) {
-    // We're likely dealing with zoneinfo that doesn't support really old
-    // timestamps, so America/New_York never looks to be on local mean time.
+    // It looks like the tzdata is only 32 bit (probably macOS),
+    // which bottoms out at 1901-12-13T20:45:52+00:00.
   } else {
     TestFormatSpecifier(tp, tz, "%E*z", "-04:56:02");
     TestFormatSpecifier(tp, tz, "%Ez", "-04:56");
@@ -464,12 +475,10 @@ TEST(Format, ExtendedSecondOffset) {
 
   EXPECT_TRUE(load_time_zone("Europe/Moscow", &tz));
   tp = convert(civil_second(1919, 6, 30, 23, 59, 59), utc);
-#if defined(__ANDROID__) && __ANDROID_API__ < 25
-  // Only Android 'N'.1 and beyond have this tz2016g transition.
-#else
-  TestFormatSpecifier(tp, tz, "%E*z", "+04:31:19");
-  TestFormatSpecifier(tp, tz, "%Ez", "+04:31");
-#endif
+  if (VersionCmp(tz, "2016g") >= 0) {
+    TestFormatSpecifier(tp, tz, "%E*z", "+04:31:19");
+    TestFormatSpecifier(tp, tz, "%Ez", "+04:31");
+  }
   tp += chrono::seconds(1);
   TestFormatSpecifier(tp, tz, "%E*z", "+04:00:00");
 }
diff --git a/absl/time/internal/cctz/src/time_zone_if.h b/absl/time/internal/cctz/src/time_zone_if.h
index f10972ae..e4bd3866 100644
--- a/absl/time/internal/cctz/src/time_zone_if.h
+++ b/absl/time/internal/cctz/src/time_zone_if.h
@@ -41,9 +41,13 @@ class TimeZoneIf {
   virtual time_zone::civil_lookup MakeTime(
       const civil_second& cs) const = 0;
 
+  virtual bool NextTransition(const time_point<seconds>& tp,
+                              time_zone::civil_transition* trans) const = 0;
+  virtual bool PrevTransition(const time_point<seconds>& tp,
+                              time_zone::civil_transition* trans) const = 0;
+
+  virtual std::string Version() const = 0;
   virtual std::string Description() const = 0;
-  virtual bool NextTransition(time_point<seconds>* tp) const = 0;
-  virtual bool PrevTransition(time_point<seconds>* tp) const = 0;
 
  protected:
   TimeZoneIf() {}
diff --git a/absl/time/internal/cctz/src/time_zone_impl.cc b/absl/time/internal/cctz/src/time_zone_impl.cc
index eb96c7ef..3062ccd3 100644
--- a/absl/time/internal/cctz/src/time_zone_impl.cc
+++ b/absl/time/internal/cctz/src/time_zone_impl.cc
@@ -83,15 +83,6 @@ bool time_zone::Impl::LoadTimeZone(const std::string& name, time_zone* tz) {
   return impl != utc_impl;
 }
 
-const time_zone::Impl& time_zone::Impl::get(const time_zone& tz) {
-  if (tz.impl_ == nullptr) {
-    // Dereferencing an implicit-UTC time_zone is expected to be
-    // rare, so we don't mind paying a small synchronization cost.
-    return *UTCImpl();
-  }
-  return *tz.impl_;
-}
-
 void time_zone::Impl::ClearTimeZoneMapTestOnly() {
   std::lock_guard<std::mutex> lock(time_zone_mutex);
   if (time_zone_map != nullptr) {
diff --git a/absl/time/internal/cctz/src/time_zone_impl.h b/absl/time/internal/cctz/src/time_zone_impl.h
index fef7f226..14965ef5 100644
--- a/absl/time/internal/cctz/src/time_zone_impl.h
+++ b/absl/time/internal/cctz/src/time_zone_impl.h
@@ -37,15 +37,15 @@ class time_zone::Impl {
   // some other kind of error occurs. Note that loading "UTC" never fails.
   static bool LoadTimeZone(const std::string& name, time_zone* tz);
 
-  // Dereferences the time_zone to obtain its Impl.
-  static const time_zone::Impl& get(const time_zone& tz);
-
   // Clears the map of cached time zones.  Primarily for use in benchmarks
   // that gauge the performance of loading/parsing the time-zone data.
   static void ClearTimeZoneMapTestOnly();
 
   // The primary key is the time-zone ID (e.g., "America/New_York").
-  const std::string& name() const { return name_; }
+  const std::string& Name() const {
+    // TODO: It would nice if the zoneinfo data included the zone name.
+    return name_;
+  }
 
   // Breaks a time_point down to civil-time components in this time zone.
   time_zone::absolute_lookup BreakTime(const time_point<seconds>& tp) const {
@@ -59,28 +59,22 @@ class time_zone::Impl {
     return zone_->MakeTime(cs);
   }
 
-  // Returns an implementation-specific description of this time zone.
-  std::string Description() const { return zone_->Description(); }
-
   // Finds the time of the next/previous offset change in this time zone.
-  //
-  // By definition, NextTransition(&tp) returns false when tp has its
-  // maximum value, and PrevTransition(&tp) returns false when tp has its
-  // mimimum value.  If the zone has no transitions, the result will also
-  // be false no matter what the argument.
-  //
-  // Otherwise, when tp has its mimimum value, NextTransition(&tp) returns
-  // true and sets tp to the first recorded transition.  Chains of calls
-  // to NextTransition()/PrevTransition() will eventually return false,
-  // but it is unspecified exactly when NextTransition(&tp) jumps to false,
-  // or what time is set by PrevTransition(&tp) for a very distant tp.
-  bool NextTransition(time_point<seconds>* tp) const {
-    return zone_->NextTransition(tp);
+  bool NextTransition(const time_point<seconds>& tp,
+                      time_zone::civil_transition* trans) const {
+    return zone_->NextTransition(tp, trans);
   }
-  bool PrevTransition(time_point<seconds>* tp) const {
-    return zone_->PrevTransition(tp);
+  bool PrevTransition(const time_point<seconds>& tp,
+                      time_zone::civil_transition* trans) const {
+    return zone_->PrevTransition(tp, trans);
   }
 
+  // Returns an implementation-defined version std::string for this time zone.
+  std::string Version() const { return zone_->Version(); }
+
+  // Returns an implementation-defined description of this time zone.
+  std::string Description() const { return zone_->Description(); }
+
  private:
   explicit Impl(const std::string& name);
   static const Impl* UTCImpl();
diff --git a/absl/time/internal/cctz/src/time_zone_info.cc b/absl/time/internal/cctz/src/time_zone_info.cc
index cdd11810..bf73635d 100644
--- a/absl/time/internal/cctz/src/time_zone_info.cc
+++ b/absl/time/internal/cctz/src/time_zone_info.cc
@@ -186,14 +186,13 @@ bool TimeZoneInfo::ResetToBuiltinUTC(const seconds& offset) {
   tt.is_dst = false;
   tt.abbr_index = 0;
 
-  // We temporarily add some redundant, contemporary (2012 through 2021)
+  // We temporarily add some redundant, contemporary (2013 through 2023)
   // transitions for performance reasons.  See TimeZoneInfo::LocalTime().
   // TODO: Fix the performance issue and remove the extra transitions.
   transitions_.clear();
   transitions_.reserve(12);
   for (const std::int_fast64_t unix_time : {
            -(1LL << 59),  // BIG_BANG
-           1325376000LL,  // 2012-01-01T00:00:00+00:00
            1356998400LL,  // 2013-01-01T00:00:00+00:00
            1388534400LL,  // 2014-01-01T00:00:00+00:00
            1420070400LL,  // 2015-01-01T00:00:00+00:00
@@ -203,6 +202,8 @@ bool TimeZoneInfo::ResetToBuiltinUTC(const seconds& offset) {
            1546300800LL,  // 2019-01-01T00:00:00+00:00
            1577836800LL,  // 2020-01-01T00:00:00+00:00
            1609459200LL,  // 2021-01-01T00:00:00+00:00
+           1640995200LL,  // 2022-01-01T00:00:00+00:00
+           1672531200LL,  // 2023-01-01T00:00:00+00:00
            2147483647LL,  // 2^31 - 1
        }) {
     Transition& tr(*transitions_.emplace(transitions_.end()));
@@ -519,6 +520,13 @@ bool TimeZoneInfo::Load(const std::string& name, ZoneInfoSource* zip) {
 
   // We don't check for EOF so that we're forwards compatible.
 
+  // If we did not find version information during the standard loading
+  // process (as of tzh_version '3' that is unsupported), then ask the
+  // ZoneInfoSource for any out-of-bound version std::string it may be privy to.
+  if (version_.empty()) {
+    version_ = zip->Version();
+  }
+
   // Trim redundant transitions. zic may have added these to work around
   // differences between the glibc and reference implementations (see
   // zic.c:dontmerge) and the Qt library (see zic.c:WORK_AROUND_QTBUG_53071).
@@ -605,6 +613,10 @@ class FileZoneInfoSource : public ZoneInfoSource {
     if (rc == 0) len_ -= offset;
     return rc;
   }
+  std::string Version() const override {
+    // TODO: It would nice if the zoneinfo data included the tzdb version.
+    return std::string();
+  }
 
  protected:
   explicit FileZoneInfoSource(
@@ -654,14 +666,15 @@ std::unique_ptr<ZoneInfoSource> FileZoneInfoSource::Open(
   return std::unique_ptr<ZoneInfoSource>(new FileZoneInfoSource(fp, length));
 }
 
-#if defined(__ANDROID__)
 class AndroidZoneInfoSource : public FileZoneInfoSource {
  public:
   static std::unique_ptr<ZoneInfoSource> Open(const std::string& name);
+  std::string Version() const override { return version_; }
 
  private:
-  explicit AndroidZoneInfoSource(FILE* fp, std::size_t len)
-      : FileZoneInfoSource(fp, len) {}
+  explicit AndroidZoneInfoSource(FILE* fp, std::size_t len, const char* vers)
+      : FileZoneInfoSource(fp, len), version_(vers) {}
+  std::string version_;
 };
 
 std::unique_ptr<ZoneInfoSource> AndroidZoneInfoSource::Open(
@@ -669,6 +682,7 @@ std::unique_ptr<ZoneInfoSource> AndroidZoneInfoSource::Open(
   // Use of the "file:" prefix is intended for testing purposes only.
   if (name.compare(0, 5, "file:") == 0) return Open(name.substr(5));
 
+#if defined(__ANDROID__)
   // See Android's libc/tzcode/bionic.cpp for additional information.
   for (const char* tzdata : {"/data/misc/zoneinfo/current/tzdata",
                              "/system/usr/share/zoneinfo/tzdata"}) {
@@ -678,6 +692,7 @@ std::unique_ptr<ZoneInfoSource> AndroidZoneInfoSource::Open(
     char hbuf[24];  // covers header.zonetab_offset too
     if (fread(hbuf, 1, sizeof(hbuf), fp.get()) != sizeof(hbuf)) continue;
     if (strncmp(hbuf, "tzdata", 6) != 0) continue;
+    const char* vers = (hbuf[11] == '\0') ? hbuf + 6 : "";
     const std::int_fast32_t index_offset = Decode32(hbuf + 12);
     const std::int_fast32_t data_offset = Decode32(hbuf + 16);
     if (index_offset < 0 || data_offset < index_offset) continue;
@@ -698,13 +713,13 @@ std::unique_ptr<ZoneInfoSource> AndroidZoneInfoSource::Open(
       if (strcmp(name.c_str(), ebuf) == 0) {
         if (fseek(fp.get(), static_cast<long>(start), SEEK_SET) != 0) break;
         return std::unique_ptr<ZoneInfoSource>(new AndroidZoneInfoSource(
-            fp.release(), static_cast<std::size_t>(length)));
+            fp.release(), static_cast<std::size_t>(length), vers));
       }
     }
   }
+#endif  // __ANDROID__
   return nullptr;
 }
-#endif
 
 }  // namespace
 
@@ -722,9 +737,7 @@ bool TimeZoneInfo::Load(const std::string& name) {
   auto zip = cctz_extension::zone_info_source_factory(
       name, [](const std::string& name) -> std::unique_ptr<ZoneInfoSource> {
         if (auto zip = FileZoneInfoSource::Open(name)) return zip;
-#if defined(__ANDROID__)
         if (auto zip = AndroidZoneInfoSource::Open(name)) return zip;
-#endif
         return nullptr;
       });
   return zip != nullptr && Load(name, zip.get());
@@ -885,17 +898,20 @@ time_zone::civil_lookup TimeZoneInfo::MakeTime(const civil_second& cs) const {
   return MakeUnique(tr->unix_time + (cs - tr->civil_sec));
 }
 
+std::string TimeZoneInfo::Version() const {
+  return version_;
+}
+
 std::string TimeZoneInfo::Description() const {
   std::ostringstream oss;
-  // TODO: It would nice if the zoneinfo data included the zone name.
-  // TODO: It would nice if the zoneinfo data included the tzdb version.
   oss << "#trans=" << transitions_.size();
   oss << " #types=" << transition_types_.size();
   oss << " spec='" << future_spec_ << "'";
   return oss.str();
 }
 
-bool TimeZoneInfo::NextTransition(time_point<seconds>* tp) const {
+bool TimeZoneInfo::NextTransition(const time_point<seconds>& tp,
+                                  time_zone::civil_transition* trans) const {
   if (transitions_.empty()) return false;
   const Transition* begin = &transitions_[0];
   const Transition* end = begin + transitions_.size();
@@ -904,22 +920,24 @@ bool TimeZoneInfo::NextTransition(time_point<seconds>* tp) const {
     // really a sentinel, not a transition.  See tz/zic.c.
     ++begin;
   }
-  std::int_fast64_t unix_time = ToUnixSeconds(*tp);
+  std::int_fast64_t unix_time = ToUnixSeconds(tp);
   const Transition target = { unix_time };
   const Transition* tr = std::upper_bound(begin, end, target,
                                           Transition::ByUnixTime());
-  if (tr != begin) {  // skip no-op transitions
-    for (; tr != end; ++tr) {
-      if (!EquivTransitions(tr[-1].type_index, tr[0].type_index)) break;
-    }
+  for (; tr != end; ++tr) {  // skip no-op transitions
+    std::uint_fast8_t prev_type_index =
+        (tr == begin) ? default_transition_type_ : tr[-1].type_index;
+    if (!EquivTransitions(prev_type_index, tr[0].type_index)) break;
   }
   // When tr == end we return false, ignoring future_spec_.
   if (tr == end) return false;
-  *tp = FromUnixSeconds(tr->unix_time);
+  trans->from = tr->prev_civil_sec + 1;
+  trans->to = tr->civil_sec;
   return true;
 }
 
-bool TimeZoneInfo::PrevTransition(time_point<seconds>* tp) const {
+bool TimeZoneInfo::PrevTransition(const time_point<seconds>& tp,
+                                  time_zone::civil_transition* trans) const {
   if (transitions_.empty()) return false;
   const Transition* begin = &transitions_[0];
   const Transition* end = begin + transitions_.size();
@@ -928,11 +946,12 @@ bool TimeZoneInfo::PrevTransition(time_point<seconds>* tp) const {
     // really a sentinel, not a transition.  See tz/zic.c.
     ++begin;
   }
-  std::int_fast64_t unix_time = ToUnixSeconds(*tp);
-  if (FromUnixSeconds(unix_time) != *tp) {
+  std::int_fast64_t unix_time = ToUnixSeconds(tp);
+  if (FromUnixSeconds(unix_time) != tp) {
     if (unix_time == std::numeric_limits<std::int_fast64_t>::max()) {
       if (end == begin) return false;  // Ignore future_spec_.
-      *tp = FromUnixSeconds((--end)->unix_time);
+      trans->from = (--end)->prev_civil_sec + 1;
+      trans->to = end->civil_sec;
       return true;
     }
     unix_time += 1;  // ceils
@@ -940,14 +959,15 @@ bool TimeZoneInfo::PrevTransition(time_point<seconds>* tp) const {
   const Transition target = { unix_time };
   const Transition* tr = std::lower_bound(begin, end, target,
                                           Transition::ByUnixTime());
-  if (tr != begin) {  // skip no-op transitions
-    for (; tr - 1 != begin; --tr) {
-      if (!EquivTransitions(tr[-2].type_index, tr[-1].type_index)) break;
-    }
+  for (; tr != begin; --tr) {  // skip no-op transitions
+    std::uint_fast8_t prev_type_index =
+        (tr - 1 == begin) ? default_transition_type_ : tr[-2].type_index;
+    if (!EquivTransitions(prev_type_index, tr[-1].type_index)) break;
   }
   // When tr == end we return the "last" transition, ignoring future_spec_.
   if (tr == begin) return false;
-  *tp = FromUnixSeconds((--tr)->unix_time);
+  trans->from = (--tr)->prev_civil_sec + 1;
+  trans->to = tr->civil_sec;
   return true;
 }
 
diff --git a/absl/time/internal/cctz/src/time_zone_info.h b/absl/time/internal/cctz/src/time_zone_info.h
index d28443e2..958e9b6b 100644
--- a/absl/time/internal/cctz/src/time_zone_info.h
+++ b/absl/time/internal/cctz/src/time_zone_info.h
@@ -74,9 +74,12 @@ class TimeZoneInfo : public TimeZoneIf {
       const time_point<seconds>& tp) const override;
   time_zone::civil_lookup MakeTime(
       const civil_second& cs) const override;
+  bool NextTransition(const time_point<seconds>& tp,
+                      time_zone::civil_transition* trans) const override;
+  bool PrevTransition(const time_point<seconds>& tp,
+                      time_zone::civil_transition* trans) const override;
+  std::string Version() const override;
   std::string Description() const override;
-  bool NextTransition(time_point<seconds>* tp) const override;
-  bool PrevTransition(time_point<seconds>* tp) const override;
 
  private:
   struct Header {  // counts of:
@@ -114,6 +117,7 @@ class TimeZoneInfo : public TimeZoneIf {
   std::uint_fast8_t default_transition_type_;  // for before first transition
   std::string abbreviations_;  // all the NUL-terminated abbreviations
 
+  std::string version_;      // the tzdata version if available
   std::string future_spec_;  // for after the last zic transition
   bool extended_;            // future_spec_ was used to generate transitions
   year_t last_year_;         // the final year of the generated transitions
diff --git a/absl/time/internal/cctz/src/time_zone_libc.cc b/absl/time/internal/cctz/src/time_zone_libc.cc
index 1d727bde..074c8d0a 100644
--- a/absl/time/internal/cctz/src/time_zone_libc.cc
+++ b/absl/time/internal/cctz/src/time_zone_libc.cc
@@ -139,16 +139,22 @@ time_zone::civil_lookup TimeZoneLibC::MakeTime(const civil_second& cs) const {
   return cl;
 }
 
-std::string TimeZoneLibC::Description() const {
-  return local_ ? "localtime" : "UTC";
+bool TimeZoneLibC::NextTransition(const time_point<seconds>& tp,
+                                  time_zone::civil_transition* trans) const {
+  return false;
 }
 
-bool TimeZoneLibC::NextTransition(time_point<seconds>* tp) const {
+bool TimeZoneLibC::PrevTransition(const time_point<seconds>& tp,
+                                  time_zone::civil_transition* trans) const {
   return false;
 }
 
-bool TimeZoneLibC::PrevTransition(time_point<seconds>* tp) const {
-  return false;
+std::string TimeZoneLibC::Version() const {
+  return std::string();  // unknown
+}
+
+std::string TimeZoneLibC::Description() const {
+  return local_ ? "localtime" : "UTC";
 }
 
 }  // namespace cctz
diff --git a/absl/time/internal/cctz/src/time_zone_libc.h b/absl/time/internal/cctz/src/time_zone_libc.h
index 4c64cd34..4e40c61a 100644
--- a/absl/time/internal/cctz/src/time_zone_libc.h
+++ b/absl/time/internal/cctz/src/time_zone_libc.h
@@ -35,9 +35,12 @@ class TimeZoneLibC : public TimeZoneIf {
       const time_point<seconds>& tp) const override;
   time_zone::civil_lookup MakeTime(
       const civil_second& cs) const override;
+  bool NextTransition(const time_point<seconds>& tp,
+                      time_zone::civil_transition* trans) const override;
+  bool PrevTransition(const time_point<seconds>& tp,
+                      time_zone::civil_transition* trans) const override;
+  std::string Version() const override;
   std::string Description() const override;
-  bool NextTransition(time_point<seconds>* tp) const override;
-  bool PrevTransition(time_point<seconds>* tp) const override;
 
  private:
   const bool local_;  // localtime or UTC
diff --git a/absl/time/internal/cctz/src/time_zone_lookup.cc b/absl/time/internal/cctz/src/time_zone_lookup.cc
index 2f6cd98b..f2d151e4 100644
--- a/absl/time/internal/cctz/src/time_zone_lookup.cc
+++ b/absl/time/internal/cctz/src/time_zone_lookup.cc
@@ -61,20 +61,43 @@ int __system_property_get(const char* name, char* value) {
 #endif
 
 std::string time_zone::name() const {
-  return time_zone::Impl::get(*this).name();
+  return effective_impl().Name();
 }
 
 time_zone::absolute_lookup time_zone::lookup(
     const time_point<seconds>& tp) const {
-  return time_zone::Impl::get(*this).BreakTime(tp);
+  return effective_impl().BreakTime(tp);
 }
 
 time_zone::civil_lookup time_zone::lookup(const civil_second& cs) const {
-  return time_zone::Impl::get(*this).MakeTime(cs);
+  return effective_impl().MakeTime(cs);
 }
 
-bool operator==(time_zone lhs, time_zone rhs) {
-  return &time_zone::Impl::get(lhs) == &time_zone::Impl::get(rhs);
+bool time_zone::next_transition(const time_point<seconds>& tp,
+                                civil_transition* trans) const {
+  return effective_impl().NextTransition(tp, trans);
+}
+
+bool time_zone::prev_transition(const time_point<seconds>& tp,
+                                civil_transition* trans) const {
+  return effective_impl().PrevTransition(tp, trans);
+}
+
+std::string time_zone::version() const {
+  return effective_impl().Version();
+}
+
+std::string time_zone::description() const {
+  return effective_impl().Description();
+}
+
+const time_zone::Impl& time_zone::effective_impl() const {
+  if (impl_ == nullptr) {
+    // Dereferencing an implicit-UTC time_zone is expected to be
+    // rare, so we don't mind paying a small synchronization cost.
+    return *time_zone::Impl::UTC().impl_;
+  }
+  return *impl_;
 }
 
 bool load_time_zone(const std::string& name, time_zone* tz) {
diff --git a/absl/time/internal/cctz/src/time_zone_lookup_test.cc b/absl/time/internal/cctz/src/time_zone_lookup_test.cc
index cd9fc236..551292fb 100644
--- a/absl/time/internal/cctz/src/time_zone_lookup_test.cc
+++ b/absl/time/internal/cctz/src/time_zone_lookup_test.cc
@@ -651,6 +651,17 @@ time_zone LoadZone(const std::string& name) {
     /* EXPECT_STREQ(zone, al.abbr); */                            \
   } while (0)
 
+// These tests sometimes run on platforms that have zoneinfo data so old
+// that the transition we are attempting to check does not exist, most
+// notably Android emulators.  Fortunately, AndroidZoneInfoSource supports
+// time_zone::version() so, in cases where we've learned that it matters,
+// we can make the check conditionally.
+int VersionCmp(time_zone tz, const std::string& target) {
+  std::string version = tz.version();
+  if (version.empty() && !target.empty()) return 1;  // unknown > known
+  return version.compare(target);
+}
+
 }  // namespace
 
 TEST(TimeZones, LoadZonesConcurrently) {
@@ -981,6 +992,69 @@ TEST(MakeTime, SysSecondsLimits) {
   EXPECT_EQ(time_point<absl::time_internal::cctz::seconds>::min(), tp);
 }
 
+TEST(NextTransition, UTC) {
+  const auto tz = utc_time_zone();
+  time_zone::civil_transition trans;
+
+  auto tp = time_point<absl::time_internal::cctz::seconds>::min();
+  EXPECT_FALSE(tz.next_transition(tp, &trans));
+
+  tp = time_point<absl::time_internal::cctz::seconds>::max();
+  EXPECT_FALSE(tz.next_transition(tp, &trans));
+}
+
+TEST(PrevTransition, UTC) {
+  const auto tz = utc_time_zone();
+  time_zone::civil_transition trans;
+
+  auto tp = time_point<absl::time_internal::cctz::seconds>::max();
+  EXPECT_FALSE(tz.prev_transition(tp, &trans));
+
+  tp = time_point<absl::time_internal::cctz::seconds>::min();
+  EXPECT_FALSE(tz.prev_transition(tp, &trans));
+}
+
+TEST(NextTransition, AmericaNewYork) {
+  const auto tz = LoadZone("America/New_York");
+  time_zone::civil_transition trans;
+
+  auto tp = convert(civil_second(2018, 6, 30, 0, 0, 0), tz);
+  EXPECT_TRUE(tz.next_transition(tp, &trans));
+  EXPECT_EQ(civil_second(2018, 11, 4, 2, 0, 0), trans.from);
+  EXPECT_EQ(civil_second(2018, 11, 4, 1, 0, 0), trans.to);
+
+  tp = time_point<absl::time_internal::cctz::seconds>::max();
+  EXPECT_FALSE(tz.next_transition(tp, &trans));
+
+  tp = time_point<absl::time_internal::cctz::seconds>::min();
+  EXPECT_TRUE(tz.next_transition(tp, &trans));
+  if (trans.from == civil_second(1918, 3, 31, 2, 0, 0)) {
+    // It looks like the tzdata is only 32 bit (probably macOS),
+    // which bottoms out at 1901-12-13T20:45:52+00:00.
+    EXPECT_EQ(civil_second(1918, 3, 31, 3, 0, 0), trans.to);
+  } else {
+    EXPECT_EQ(civil_second(1883, 11, 18, 12, 3, 58), trans.from);
+    EXPECT_EQ(civil_second(1883, 11, 18, 12, 0, 0), trans.to);
+  }
+}
+
+TEST(PrevTransition, AmericaNewYork) {
+  const auto tz = LoadZone("America/New_York");
+  time_zone::civil_transition trans;
+
+  auto tp = convert(civil_second(2018, 6, 30, 0, 0, 0), tz);
+  EXPECT_TRUE(tz.prev_transition(tp, &trans));
+  EXPECT_EQ(civil_second(2018, 3, 11, 2, 0, 0), trans.from);
+  EXPECT_EQ(civil_second(2018, 3, 11, 3, 0, 0), trans.to);
+
+  tp = time_point<absl::time_internal::cctz::seconds>::min();
+  EXPECT_FALSE(tz.prev_transition(tp, &trans));
+
+  tp = time_point<absl::time_internal::cctz::seconds>::max();
+  EXPECT_TRUE(tz.prev_transition(tp, &trans));
+  // We have a transition but we don't know which one.
+}
+
 TEST(TimeZoneEdgeCase, AmericaNewYork) {
   const time_zone tz = LoadZone("America/New_York");
 
@@ -1104,35 +1178,31 @@ TEST(TimeZoneEdgeCase, PacificApia) {
 TEST(TimeZoneEdgeCase, AfricaCairo) {
   const time_zone tz = LoadZone("Africa/Cairo");
 
-#if defined(__ANDROID__) && __ANDROID_API__ < 21
-  // Only Android 'L' and beyond have this tz2014c transition.
-#else
-  // An interesting case of midnight not existing.
-  //
-  //   1400191199 == Thu, 15 May 2014 23:59:59 +0200 (EET)
-  //   1400191200 == Fri, 16 May 2014 01:00:00 +0300 (EEST)
-  auto tp = convert(civil_second(2014, 5, 15, 23, 59, 59), tz);
-  ExpectTime(tp, tz, 2014, 5, 15, 23, 59, 59, 2 * 3600, false, "EET");
-  tp += absl::time_internal::cctz::seconds(1);
-  ExpectTime(tp, tz, 2014, 5, 16, 1, 0, 0, 3 * 3600, true, "EEST");
-#endif
+  if (VersionCmp(tz, "2014c") >= 0) {
+    // An interesting case of midnight not existing.
+    //
+    //   1400191199 == Thu, 15 May 2014 23:59:59 +0200 (EET)
+    //   1400191200 == Fri, 16 May 2014 01:00:00 +0300 (EEST)
+    auto tp = convert(civil_second(2014, 5, 15, 23, 59, 59), tz);
+    ExpectTime(tp, tz, 2014, 5, 15, 23, 59, 59, 2 * 3600, false, "EET");
+    tp += absl::time_internal::cctz::seconds(1);
+    ExpectTime(tp, tz, 2014, 5, 16, 1, 0, 0, 3 * 3600, true, "EEST");
+  }
 }
 
 TEST(TimeZoneEdgeCase, AfricaMonrovia) {
   const time_zone tz = LoadZone("Africa/Monrovia");
 
-#if defined(__ANDROID__) && __ANDROID_API__ < 26
-  // Only Android 'O' and beyond have this tz2017b transition.
-#else
-  // Strange offset change -00:44:30 -> +00:00:00 (non-DST)
-  //
-  //   63593069 == Thu,  6 Jan 1972 23:59:59 -0044 (MMT)
-  //   63593070 == Fri,  7 Jan 1972 00:44:30 +0000 (GMT)
-  auto tp = convert(civil_second(1972, 1, 6, 23, 59, 59), tz);
-  ExpectTime(tp, tz, 1972, 1, 6, 23, 59, 59, -44.5 * 60, false, "MMT");
-  tp += absl::time_internal::cctz::seconds(1);
-  ExpectTime(tp, tz, 1972, 1, 7, 0, 44, 30, 0 * 60, false, "GMT");
-#endif
+  if (VersionCmp(tz, "2017b") >= 0) {
+    // Strange offset change -00:44:30 -> +00:00:00 (non-DST)
+    //
+    //   63593069 == Thu,  6 Jan 1972 23:59:59 -0044 (MMT)
+    //   63593070 == Fri,  7 Jan 1972 00:44:30 +0000 (GMT)
+    auto tp = convert(civil_second(1972, 1, 6, 23, 59, 59), tz);
+    ExpectTime(tp, tz, 1972, 1, 6, 23, 59, 59, -44.5 * 60, false, "MMT");
+    tp += absl::time_internal::cctz::seconds(1);
+    ExpectTime(tp, tz, 1972, 1, 7, 0, 44, 30, 0 * 60, false, "GMT");
+  }
 }
 
 TEST(TimeZoneEdgeCase, AmericaJamaica) {
@@ -1144,28 +1214,29 @@ TEST(TimeZoneEdgeCase, AmericaJamaica) {
   const time_zone tz = LoadZone("America/Jamaica");
 
   // Before the first transition.
-  auto tp = convert(civil_second(1889, 12, 31, 0, 0, 0), tz);
-#if AMERICA_JAMAICA_PRE_1913_OFFSET_FIX
-  // Commit 907241e: Fix off-by-1 error for Jamaica and T&C before 1913.
-  // Until that commit has made its way into a full release we avoid the
-  // expectations on the -18430 offset below.  TODO: Uncomment these.
-  ExpectTime(tp, tz, 1889, 12, 31, 0, 0, 0, -18430, false,
-             tz.lookup(tp).abbr);
-
-  // Over the first (abbreviation-change only) transition.
-  //   -2524503170 == Tue, 31 Dec 1889 23:59:59 -0507 (LMT)
-  //   -2524503169 == Wed,  1 Jan 1890 00:00:00 -0507 (KMT)
-  tp = convert(civil_second(1889, 12, 31, 23, 59, 59), tz);
-  ExpectTime(tp, tz, 1889, 12, 31, 23, 59, 59, -18430, false,
-             tz.lookup(tp).abbr);
-  tp += absl::time_internal::cctz::seconds(1);
-  ExpectTime(tp, tz, 1890, 1, 1, 0, 0, 0, -18430, false, "KMT");
-#endif
+  if (!tz.version().empty() && VersionCmp(tz, "2018d") >= 0) {
+    // We avoid the expectations on the -18430 offset below unless we are
+    // certain we have commit 907241e (Fix off-by-1 error for Jamaica and
+    // T&C before 1913) from 2018d.  TODO: Remove the "version() not empty"
+    // part when 2018d is generally available from /usr/share/zoneinfo.
+    auto tp = convert(civil_second(1889, 12, 31, 0, 0, 0), tz);
+    ExpectTime(tp, tz, 1889, 12, 31, 0, 0, 0, -18430, false,
+               tz.lookup(tp).abbr);
+
+    // Over the first (abbreviation-change only) transition.
+    //   -2524503170 == Tue, 31 Dec 1889 23:59:59 -0507 (LMT)
+    //   -2524503169 == Wed,  1 Jan 1890 00:00:00 -0507 (KMT)
+    tp = convert(civil_second(1889, 12, 31, 23, 59, 59), tz);
+    ExpectTime(tp, tz, 1889, 12, 31, 23, 59, 59, -18430, false,
+               tz.lookup(tp).abbr);
+    tp += absl::time_internal::cctz::seconds(1);
+    ExpectTime(tp, tz, 1890, 1, 1, 0, 0, 0, -18430, false, "KMT");
+  }
 
   // Over the last (DST) transition.
   //     436341599 == Sun, 30 Oct 1983 01:59:59 -0400 (EDT)
   //     436341600 == Sun, 30 Oct 1983 01:00:00 -0500 (EST)
-  tp = convert(civil_second(1983, 10, 30, 1, 59, 59), tz);
+  auto tp = convert(civil_second(1983, 10, 30, 1, 59, 59), tz);
   ExpectTime(tp, tz, 1983, 10, 30, 1, 59, 59, -4 * 3600, true, "EDT");
   tp += absl::time_internal::cctz::seconds(1);
   ExpectTime(tp, tz, 1983, 10, 30, 1, 0, 0, -5 * 3600, false, "EST");
diff --git a/absl/time/internal/cctz/src/zone_info_source.cc b/absl/time/internal/cctz/src/zone_info_source.cc
index ee7500b6..bf2d2d2d 100644
--- a/absl/time/internal/cctz/src/zone_info_source.cc
+++ b/absl/time/internal/cctz/src/zone_info_source.cc
@@ -20,6 +20,7 @@ namespace cctz {
 
 // Defined out-of-line to avoid emitting a weak vtable in all TUs.
 ZoneInfoSource::~ZoneInfoSource() {}
+std::string ZoneInfoSource::Version() const { return std::string(); }
 
 }  // namespace cctz
 }  // namespace time_internal