From 1eed461d06b02854d9310d5ab0198eccd2be1d1d Mon Sep 17 00:00:00 2001
From: Howard Hinnant <howard.hinnant@gmail.com>
Date: Wed, 3 Oct 2018 17:51:55 -0400
Subject: [PATCH] Model the TAI-UTC difference between 1961 and 1972

---
 include/date/tz.h                     | 103 +++++++++++++++++++++++++-
 test/clock_cast_test/local_t.pass.cpp |   2 +-
 2 files changed, 102 insertions(+), 3 deletions(-)

diff --git a/include/date/tz.h b/include/date/tz.h
index 49e4320..1eba540 100644
--- a/include/date/tz.h
+++ b/include/date/tz.h
@@ -2057,6 +2057,73 @@ template <class Duration>
 
 using tai_seconds = tai_time<std::chrono::seconds>;
 
+namespace detail
+{
+
+template <class Duration>
+inline
+std::chrono::duration<long double, days::period>
+mjd(sys_time<Duration> tp)
+{
+    using namespace std::chrono;
+//     constexpr auto off = sys_days{1970_y/1/1} - (sys_days{-4713_y/11/24} + hours{12}) -
+//                          dday{2400000.5};
+    constexpr days off{40587};
+    return tp.time_since_epoch() + off;
+}
+
+template <class Duration>
+Duration
+pre_leap(sys_time<Duration> tp)
+{
+    using namespace std::chrono;
+    using dsec = duration<long double>;
+    if (tp >= sys_days(1972_y/January/1))
+        return seconds{10};
+    if (tp < sys_days(1961_y/January/1))
+        return Duration{0};
+    if (tp < sys_days(1961_y/August/1))
+        return duration_cast<Duration>(
+                   dsec{1.422818} + (mjd(tp) - days{37300}).count() * dsec{0.001296});
+    if (tp < sys_days(1962_y/January/1))
+        return duration_cast<Duration>(
+                   dsec{1.372818} + (mjd(tp) - days{37300}).count() * dsec{0.001296});
+    if (tp < sys_days(1963_y/November/1))
+        return duration_cast<Duration>(
+                   dsec{1.845858} + (mjd(tp) - days{37665}).count() * dsec{0.0011232});
+    if (tp < sys_days(1964_y/January/1))
+        return duration_cast<Duration>(
+                   dsec{1.945858} + (mjd(tp) - days{37665}).count() * dsec{0.0011232});
+    if (tp < sys_days(1964_y/April/1))
+        return duration_cast<Duration>(
+                   dsec{3.240130} + (mjd(tp) - days{38761}).count() * dsec{0.001296});
+    if (tp < sys_days(1964_y/September/1))
+        return duration_cast<Duration>(
+                   dsec{3.340130} + (mjd(tp) - days{38761}).count() * dsec{0.001296});
+    if (tp < sys_days(1965_y/January/1))
+        return duration_cast<Duration>(
+                   dsec{3.440130} + (mjd(tp) - days{38761}).count() * dsec{0.001296});
+    if (tp < sys_days(1965_y/March/1))
+        return duration_cast<Duration>(
+                   dsec{3.540130} + (mjd(tp) - days{38761}).count() * dsec{0.001296});
+    if (tp < sys_days(1965_y/July/1))
+        return duration_cast<Duration>(
+                   dsec{3.640130} + (mjd(tp) - days{38761}).count() * dsec{0.001296});
+    if (tp < sys_days(1965_y/September/1))
+        return duration_cast<Duration>(
+                   dsec{3.740130} + (mjd(tp) - days{38761}).count() * dsec{0.001296});
+    if (tp < sys_days(1966_y/January/1))
+        return duration_cast<Duration>(
+                   dsec{3.840130} + (mjd(tp) - days{38761}).count() * dsec{0.001296});
+    if (tp < sys_days(1968_y/February/1))
+        return duration_cast<Duration>(
+                   dsec{4.313170} + (mjd(tp) - days{39126}).count() * dsec{0.002592});
+    return duration_cast<Duration>(
+               dsec{4.213170} + (mjd(tp) - days{39126}).count() * dsec{0.002592});
+}
+
+}  // namespace detail
+
 template <class Duration>
 inline
 utc_time<typename std::common_type<Duration, std::chrono::seconds>::type>
@@ -2065,7 +2132,8 @@ tai_clock::to_utc(const tai_time<Duration>& t) NOEXCEPT
     using namespace std::chrono;
     using CD = typename std::common_type<Duration, seconds>::type;
     return utc_time<CD>{t.time_since_epoch()} -
-            (sys_days(year{1970}/January/1) - sys_days(year{1958}/January/1) + seconds{10});
+            (sys_days(year{1970}/January/1) - sys_days(year{1958}/January/1) +
+             detail::pre_leap<CD>(sys_time<CD>(to_local(t).time_since_epoch())));
 }
 
 template <class Duration>
@@ -2076,7 +2144,8 @@ tai_clock::from_utc(const utc_time<Duration>& t) NOEXCEPT
     using namespace std::chrono;
     using CD = typename std::common_type<Duration, seconds>::type;
     return tai_time<CD>{t.time_since_epoch()} +
-            (sys_days(year{1970}/January/1) - sys_days(year{1958}/January/1) + seconds{10});
+            (sys_days(year{1970}/January/1) - sys_days(year{1958}/January/1) +
+             detail::pre_leap<CD>(sys_time<CD>(t.time_since_epoch())));
 }
 
 inline
@@ -2178,6 +2247,34 @@ template <class Duration>
 
 using gps_seconds = gps_time<std::chrono::seconds>;
 
+#if 1
+
+template <class Duration>
+inline
+utc_time<typename std::common_type<Duration, std::chrono::seconds>::type>
+gps_clock::to_utc(const gps_time<Duration>& t) NOEXCEPT
+{
+    using namespace std::chrono;
+    using CD = typename std::common_type<Duration, seconds>::type;
+    return tai_clock::to_utc(tai_time<CD>{t.time_since_epoch()} +
+            (sys_days(year{1980}/January/Sunday[1]) - sys_days(year{1958}/January/1) +
+             seconds{19}));
+}
+
+template <class Duration>
+inline
+gps_time<typename std::common_type<Duration, std::chrono::seconds>::type>
+gps_clock::from_utc(const utc_time<Duration>& t) NOEXCEPT
+{
+    using namespace std::chrono;
+    using CD = typename std::common_type<Duration, seconds>::type;
+    return gps_time<CD>{tai_clock::from_utc(t).time_since_epoch()} -
+            (sys_days(year{1980}/January/Sunday[1]) - sys_days(year{1958}/January/1) +
+             seconds{19});
+}
+
+#else
+
 template <class Duration>
 inline
 utc_time<typename std::common_type<Duration, std::chrono::seconds>::type>
@@ -2202,6 +2299,8 @@ gps_clock::from_utc(const utc_time<Duration>& t) NOEXCEPT
              seconds{9});
 }
 
+#endif
+
 inline
 gps_clock::time_point
 gps_clock::now()
diff --git a/test/clock_cast_test/local_t.pass.cpp b/test/clock_cast_test/local_t.pass.cpp
index c80459d..a1e5de6 100644
--- a/test/clock_cast_test/local_t.pass.cpp
+++ b/test/clock_cast_test/local_t.pass.cpp
@@ -82,7 +82,7 @@ main()
      assert(clock_cast<local_t>(tt) == lt);
      assert(tt.time_since_epoch() == seconds(0));
 
-     auto lu = local_days{1958_y/January/1_d} - seconds(10);
+     auto lu = local_days{1958_y/January/1_d};
      auto ut = clock_cast<utc_clock>(lu);
      assert(clock_cast<tai_clock>(ut) == tt);
    }