calendar-intervalcalendar-interval is a compiled Lisp function in `
calendar.el'. (calendar-interval MON1 YR1 MON2 YR2) The number of months difference between MON1, YR1 and MON2, YR2. The result is positive if the second date is later than the first. Negative years are interpreted as years BC; -1 being 1 BC, and so on.