diff options
| -rw-r--r-- | sfeed.c | 17 |
1 files changed, 14 insertions, 3 deletions
@@ -432,10 +432,13 @@ string_print_timestamp(String *s) printf("%lld", t); } -/* Convert time fields. Returns a UNIX timestamp. */ +/* Convert time fields. Returns a signed (atleast) 64-bit UNIX timestamp. + Parameters should be passed as they are in a struct tm: + that is: year = year - 1900, month = month - 1. */ static long long datetounix(long long year, int mon, int day, int hour, int min, int sec) { + /* seconds in a month in a regular (non-leap) year */ static const long secs_through_month[] = { 0, 31 * 86400, 59 * 86400, 90 * 86400, 120 * 86400, 151 * 86400, 181 * 86400, 212 * 86400, @@ -443,7 +446,9 @@ datetounix(long long year, int mon, int day, int hour, int min, int sec) int is_leap = 0, cycles, centuries = 0, leaps = 0, rem; long long t; + /* optimization: handle common range year 1902 up to and including 2038 */ if (year - 2ULL <= 136) { + /* amount of leap days relative to 1970: every 4 years */ leaps = (year - 68) >> 2; if (!((year - 68) & 3)) { leaps--; @@ -451,8 +456,11 @@ datetounix(long long year, int mon, int day, int hour, int min, int sec) } else { is_leap = 0; } - t = 31536000 * (year - 70) + 86400 * leaps; + t = 31536000 * (year - 70) + (86400 * leaps); /* 365 * 86400 = 31536000 */ } else { + /* general leap year calculation: + leap years occur mostly every 4 years but every 100 years + a leap year is skipped unless the year is divisible by 400 */ cycles = (year - 100) / 400; rem = (year - 100) % 400; if (rem < 0) { @@ -474,7 +482,10 @@ datetounix(long long year, int mon, int day, int hour, int min, int sec) is_leap = !rem; } } - leaps += 97 * cycles + 24 * centuries - is_leap; + leaps += (97 * cycles) + (24 * centuries) - is_leap; + + /* adjust 8 leap days from 1970 up to and including 2000: + ((30 * 365) + 8) * 86400 = 946771200 */ t = ((year - 100) * 31536000LL) + (leaps * 86400LL) + 946771200LL; } t += secs_through_month[mon]; |