rem: tm_diff (a, b) const struct tm *a; const struct tm *b;
add: tm_diff (const struct tm *a, const struct tm *b)
context: tm_diff (a, b)     const struct tm *a;     const struct tm *b;{  /* 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));  return (60 * (60 * (24 * days + (a->tm_hour - b->tm_hour))		+ (a->tm_min - b->tm_min))	  + (a->tm_sec - b->tm_sec));}