Merge pull request #262 from rieck/timestamp-fix

Fix for incorrect conversion from `watch_date_time` to Unix time.

1 files changed, 73 insertions, 5 deletions

diff --git a/watch-library/shared/watch/watch_utility.c b/watch-library/shared/watch/watch_utility.c
index 9e524762..64b3bb79 100644
--- a/watch-library/shared/watch/watch_utility.c
+++ b/watch-library/shared/watch/watch_utility.c
@@ -102,13 +102,81 @@ uint16_t watch_utility_days_since_new_year(uint16_t year, uint8_t month, uint8_t
     return (is_leap(year) && (month > 2) ? 1 : 0) + DAYS_SO_FAR[month - 1] + day;
 }
 
+// Function taken from `src/time/__year_to_secs.c` of musl libc
+// https://musl.libc.org
+static uint32_t __year_to_secs(uint32_t year, int *is_leap)
+{
+	if (year-2ULL <= 136) {
+		int y = year;
+		int leaps = (y-68)>>2;
+		if (!((y-68)&3)) {
+			leaps--;
+			if (is_leap) *is_leap = 1;
+		} else if (is_leap) *is_leap = 0;
+		return 31536000*(y-70) + 86400*leaps;
+	}
+
+	int cycles, centuries, leaps, rem;
+
+	if (!is_leap) is_leap = &(int){0};
+	cycles = (year-100) / 400;
+	rem = (year-100) % 400;
+	if (rem < 0) {
+		cycles--;
+		rem += 400;
+	}
+	if (!rem) {
+		*is_leap = 1;
+		centuries = 0;
+		leaps = 0;
+	} else {
+		if (rem >= 200) {
+			if (rem >= 300) centuries = 3, rem -= 300;
+			else centuries = 2, rem -= 200;
+		} else {
+			if (rem >= 100) centuries = 1, rem -= 100;
+			else centuries = 0;
+		}
+		if (!rem) {
+			*is_leap = 0;
+			leaps = 0;
+		} else {
+			leaps = rem / 4U;
+			rem %= 4U;
+			*is_leap = !rem;
+		}
+	}
+
+	leaps += 97*cycles + 24*centuries - *is_leap;
+
+	return (year-100) * 31536000LL + leaps * 86400LL + 946684800 + 86400;
+}
+
+// Function taken from `src/time/__month_to_secs.c` of musl libc
+// https://musl.libc.org
+static int __month_to_secs(int month, int is_leap)
+{
+	static const int secs_through_month[] = {
+		0, 31*86400, 59*86400, 90*86400,
+		120*86400, 151*86400, 181*86400, 212*86400,
+		243*86400, 273*86400, 304*86400, 334*86400 };
+	int t = secs_through_month[month];
+	if (is_leap && month >= 2) t+=86400;
+	return t;
+}
+
+// Function adapted from `src/time/__tm_to_secs.c` of musl libc
+// https://musl.libc.org
 uint32_t watch_utility_convert_to_unix_time(uint16_t year, uint8_t month, uint8_t day, uint8_t hour, uint8_t minute, uint8_t second, uint32_t utc_offset) {
-    uint32_t year_adj = year + 4800;
-    uint32_t leap_days = 1 + (year_adj / 4) - (year_adj / 100) + (year_adj / 400);
-    uint32_t days = 365 * year_adj + leap_days + watch_utility_days_since_new_year(year, month, day) - 1;
-    days -= 2472692;  /* Adjust to Unix epoch. */
+    int is_leap;
+
+    // POSIX tm struct starts year at 1900 and month at 0
+    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html 
+    uint32_t timestamp = __year_to_secs(year - 1900, &is_leap);
+    timestamp += __month_to_secs(month - 1, is_leap);
 
-    uint32_t timestamp = days * 86400;
+    // Regular conversion from musl libc
+    timestamp += (day - 1) * 86400;
     timestamp += hour * 3600;
     timestamp += minute * 60;
     timestamp += second;