days_in_mon
day1 = next_ge((d1-1)%days_in_mon(tm->tm_mon, tm->tm_year) + 1,
day2 = (d1 + daysahead-1)%days_in_mon(tm->tm_mon, tm->tm_year) + 1;
d1 = days_in_mon(mon, yr);
extern int days_in_mon(int, int);
if (d2 > days_in_mon(m2, y2)) {
days = (days_in_mon(m1, y1)-d1) + (d2-1);
days += days_in_mon(m, y1);