increment_overflow
static int increment_overflow(int * number, int delta);
if (increment_overflow(&newy, idelta))
if (increment_overflow(&y, -1))
if (increment_overflow(&y, 1))
if (increment_overflow(&tmp->tm_year, -TM_YEAR_BASE))
return increment_overflow(tensptr, tensdelta);
if (increment_overflow(&yourtm.tm_sec, 1 - SECSPERMIN))