[time] Use O(1) closed-form leap year math for epoch-to-year conversion (#15368)

2026-05-23 03:06:05 +08:00 · 2026-04-02 09:19:47 -10:00
parent e7e590b36f
commit da09e1e1ce
2 changed files with 144 additions and 28 deletions
@@ -758,6 +758,115 @@ TEST(PosixTzParser, EpochToLocalDstTransition) {
  EXPECT_EQ(local.tm_isdst, 1);
 }

+// ============================================================================
+// Leap year edge cases for closed-form year arithmetic
+// ============================================================================
+
+TEST(PosixTzParser, EpochToLocalLeapYear2000) {
+  // 2000 is a leap year (divisible by 400)
+  ParsedTimezone tz;
+  ASSERT_TRUE(parse_posix_tz("UTC0", tz));
+
+  // Feb 29, 2000 12:00:00 UTC
+  time_t epoch = make_utc(2000, 2, 29, 12);
+  struct tm local;
+  ASSERT_TRUE(epoch_to_local_tm(epoch, tz, &local));
+  EXPECT_EQ(local.tm_year, 100);  // 2000
+  EXPECT_EQ(local.tm_mon, 1);     // February
+  EXPECT_EQ(local.tm_mday, 29);
+  EXPECT_EQ(local.tm_hour, 12);
+}
+
+TEST(PosixTzParser, EpochToLocalNonLeapYear2100) {
+  // 2100 is NOT a leap year (divisible by 100 but not 400)
+  ParsedTimezone tz;
+  ASSERT_TRUE(parse_posix_tz("UTC0", tz));
+
+  // Mar 1, 2100 00:00:00 UTC — the day after what would be Feb 29
+  time_t epoch = make_utc(2100, 3, 1);
+  struct tm local;
+  ASSERT_TRUE(epoch_to_local_tm(epoch, tz, &local));
+  EXPECT_EQ(local.tm_year, 200);  // 2100
+  EXPECT_EQ(local.tm_mon, 2);     // March
+  EXPECT_EQ(local.tm_mday, 1);
+
+  // Feb 28, 2100 23:59:59 UTC — last second of February (no Feb 29)
+  epoch = make_utc(2100, 2, 28, 23, 59, 59);
+  ASSERT_TRUE(epoch_to_local_tm(epoch, tz, &local));
+  EXPECT_EQ(local.tm_year, 200);
+  EXPECT_EQ(local.tm_mon, 1);  // February
+  EXPECT_EQ(local.tm_mday, 28);
+}
+
+TEST(PosixTzParser, EpochToLocalLeapYear2400) {
+  // 2400 is a leap year (divisible by 400)
+  ParsedTimezone tz;
+  ASSERT_TRUE(parse_posix_tz("UTC0", tz));
+
+  time_t epoch = make_utc(2400, 2, 29, 6);
+  struct tm local;
+  ASSERT_TRUE(epoch_to_local_tm(epoch, tz, &local));
+  EXPECT_EQ(local.tm_year, 500);  // 2400
+  EXPECT_EQ(local.tm_mon, 1);     // February
+  EXPECT_EQ(local.tm_mday, 29);
+  EXPECT_EQ(local.tm_hour, 6);
+}
+
+TEST(PosixTzParser, EpochToLocalNewYearBoundaries) {
+  // Test year boundary — last second of 2099 and first second of 2100
+  ParsedTimezone tz;
+  ASSERT_TRUE(parse_posix_tz("UTC0", tz));
+  struct tm local;
+
+  // Dec 31, 2099 23:59:59 UTC
+  time_t epoch = make_utc(2099, 12, 31, 23, 59, 59);
+  ASSERT_TRUE(epoch_to_local_tm(epoch, tz, &local));
+  EXPECT_EQ(local.tm_year, 199);  // 2099
+  EXPECT_EQ(local.tm_mon, 11);    // December
+  EXPECT_EQ(local.tm_mday, 31);
+
+  // Jan 1, 2100 00:00:00 UTC
+  epoch = make_utc(2100, 1, 1);
+  ASSERT_TRUE(epoch_to_local_tm(epoch, tz, &local));
+  EXPECT_EQ(local.tm_year, 200);  // 2100
+  EXPECT_EQ(local.tm_mon, 0);     // January
+  EXPECT_EQ(local.tm_mday, 1);
+}
+
+TEST(PosixTzParser, EpochToLocalDstAcrossCenturyBoundary) {
+  // DST transition in year 2100 (non-leap) with US Eastern rules
+  ParsedTimezone tz;
+  ASSERT_TRUE(parse_posix_tz("EST5EDT,M3.2.0/2,M11.1.0/2", tz));
+
+  // July 4, 2100 16:00 UTC = 12:00 EDT
+  time_t epoch = make_utc(2100, 7, 4, 16);
+  struct tm local;
+  ASSERT_TRUE(epoch_to_local_tm(epoch, tz, &local));
+  EXPECT_EQ(local.tm_hour, 12);
+  EXPECT_EQ(local.tm_isdst, 1);
+
+  // Jan 15, 2100 10:00 UTC = 05:00 EST
+  epoch = make_utc(2100, 1, 15, 10);
+  ASSERT_TRUE(epoch_to_local_tm(epoch, tz, &local));
+  EXPECT_EQ(local.tm_hour, 5);
+  EXPECT_EQ(local.tm_isdst, 0);
+}
+
+TEST(PosixTzParser, EpochToLocalFarFutureYear5000) {
+  // Year 5000 — days/365 estimate overshoots by ~2 years due to leap days,
+  // requiring multiple correction steps in days_to_year.
+  ParsedTimezone tz;
+  ASSERT_TRUE(parse_posix_tz("UTC0", tz));
+
+  time_t epoch = make_utc(5000, 6, 15, 12);
+  struct tm local;
+  ASSERT_TRUE(epoch_to_local_tm(epoch, tz, &local));
+  EXPECT_EQ(local.tm_year, 3100);  // 5000
+  EXPECT_EQ(local.tm_mon, 5);      // June
+  EXPECT_EQ(local.tm_mday, 15);
+  EXPECT_EQ(local.tm_hour, 12);
+}
+
 // ============================================================================
 // Verification against libc
 // ============================================================================