+#define TM_YEAR_ORIGIN 1900
+
+/* Yield A - B, measured in seconds. */
+static long
+difftm (a, b)
+ struct tm *a, *b;
+{
+ 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 */
+ + (long)(ay-by) * 365
+ );
+ return (60*(60*(24*days + (a->tm_hour - b->tm_hour))
+ + (a->tm_min - b->tm_min))
+ + (a->tm_sec - b->tm_sec));
+}