From 01614b40228b416fffa620e3fffe1376dec089ab Mon Sep 17 00:00:00 2001
From: Gaspard Petit <gpetit@eidosmontreal.com>
Date: Fri, 24 Feb 2023 22:54:57 -0500
Subject: [PATCH] Increase the precision when converting milliseconds to years

With the current implementation, a large negative millisecond timestamp will result in a relatively normal positive year (int64::min translates into 1535). This change improves the precision for large number of days obtained from large millisecond values.
---
 include/date/date.h         | 14 +++++++-------
 test/date_test/day.pass.cpp | 16 ++++++++++++++++
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/include/date/date.h b/include/date/date.h
index 6960e8cd..e8d02dbe 100644
--- a/include/date/date.h
+++ b/include/date/date.h
@@ -174,7 +174,7 @@ using ratio_divide = decltype(std::ratio_divide<R1, R2>{});
 // durations
 
 using days = std::chrono::duration
-    <int, detail::ratio_multiply<std::ratio<24>, std::chrono::hours::period>>;
+    <int64_t, detail::ratio_multiply<std::ratio<24>, std::chrono::hours::period>>;
 
 using weeks = std::chrono::duration
     <int, detail::ratio_multiply<std::ratio<7>, days::period>>;
@@ -394,7 +394,7 @@ operator<<(std::basic_ostream<CharT, Traits>& os, const month& m);
 
 class year
 {
-    short y_;
+    int y_;
 
 public:
     year() = default;
@@ -464,7 +464,7 @@ class weekday
     CONSTCD11 weekday_last    operator[](last_spec)      const NOEXCEPT;
 
 private:
-    static CONSTCD14 unsigned char weekday_from_days(int z) NOEXCEPT;
+    static CONSTCD14 unsigned char weekday_from_days(int64_t z) NOEXCEPT;
 
     friend CONSTCD11 bool operator==(const weekday& x, const weekday& y) NOEXCEPT;
     friend CONSTCD14 days operator-(const weekday& x, const weekday& y) NOEXCEPT;
@@ -1478,7 +1478,7 @@ inline
 days
 operator-(const day& x, const day& y) NOEXCEPT
 {
-    return days{static_cast<days::rep>(static_cast<unsigned>(x)
+    return days{static_cast<int>(static_cast<unsigned>(x)
                                      - static_cast<unsigned>(y))};
 }
 
@@ -1820,9 +1820,9 @@ operator<<(std::basic_ostream<CharT, Traits>& os, const year& y)
 CONSTCD14
 inline
 unsigned char
-weekday::weekday_from_days(int z) NOEXCEPT
+weekday::weekday_from_days(int64_t z) NOEXCEPT
 {
-    auto u = static_cast<unsigned>(z);
+    auto u = static_cast<uint64_t>(z);
     return static_cast<unsigned char>(z >= -4 ? (u+4) % 7 : u % 7);
 }
 
@@ -3099,7 +3099,7 @@ year_month_day::from_days(days dp) NOEXCEPT
     auto const mp = (5*doy + 2)/153;                                   // [0, 11]
     auto const d = doy - (153*mp+2)/5 + 1;                             // [1, 31]
     auto const m = mp < 10 ? mp+3 : mp-9;                              // [1, 12]
-    return year_month_day{date::year{y + (m <= 2)}, date::month(m), date::day(d)};
+    return year_month_day{date::year{static_cast<int>(y + (m <= 2))}, date::month(m), date::day(d)};
 }
 
 template<class>
diff --git a/test/date_test/day.pass.cpp b/test/date_test/day.pass.cpp
index 0dbdf688..f62c7025 100644
--- a/test/date_test/day.pass.cpp
+++ b/test/date_test/day.pass.cpp
@@ -140,4 +140,20 @@ main()
     os.str("");
     os << d;
     assert(os.str() == "12");
+
+    {
+        ::date::sys_time<std::chrono::milliseconds> timePoint{ std::chrono::milliseconds(0) };
+	    std::chrono::time_point<std::chrono::system_clock, ::date::days> dayPoint = ::date::floor<::date::days>(timePoint);
+	    ::date::fields<std::chrono::milliseconds> dateFields{ ::date::year_month_day{ dayPoint }, ::date::time_of_day<std::chrono::milliseconds>{ timePoint - dayPoint } };
+	    int year = (int16_t)static_cast<int>(dateFields.ymd.year());
+        assert(year == 1970);
+    }
+
+    {
+        ::date::sys_time<std::chrono::milliseconds> timePoint{ std::chrono::milliseconds(std::numeric_limits<int64_t>::min()) };
+	    std::chrono::time_point<std::chrono::system_clock, ::date::days> dayPoint = ::date::floor<::date::days>(timePoint);
+	    ::date::fields<std::chrono::milliseconds> dateFields{ ::date::year_month_day{ dayPoint }, ::date::time_of_day<std::chrono::milliseconds>{ timePoint - dayPoint } };
+	    int year = static_cast<int>(dateFields.ymd.year());
+        assert(year == -292275055);
+    }
 }