- int ay = a->tm_year + (TM_YEAR_ORIGIN - 1);
- int by = b->tm_year + (TM_YEAR_ORIGIN - 1);
- int days = (
- /* difference in day of year */
- a->tm_yday - b->tm_yday
- /* + intervening leap days */
- + ((ay >> 2) - (by >> 2))
- - (ay / 100 - by / 100)
- + ((ay / 100 >> 2) - (by / 100 >> 2))
- /* + difference in years * 365 */
- + (int) (ay - by) * 365
- );
+ /* Compute intervening leap days correctly even if year is negative.
+ Take care to avoid int overflow in leap day calculations,
+ but it's OK to assume that A and B are close to each other. */
+ int a4 = (a->tm_year >> 2) + (TM_YEAR_BASE >> 2) - ! (a->tm_year & 3);
+ int b4 = (b->tm_year >> 2) + (TM_YEAR_BASE >> 2) - ! (b->tm_year & 3);
+ int a100 = a4 / 25 - (a4 % 25 < 0);
+ int b100 = b4 / 25 - (b4 % 25 < 0);
+ int a400 = a100 >> 2;
+ int b400 = b100 >> 2;
+ int intervening_leap_days = (a4 - b4) - (a100 - b100) + (a400 - b400);
+ int years = a->tm_year - b->tm_year;
+ int days = (365 * years + intervening_leap_days
+ + (a->tm_yday - b->tm_yday));