calendar-increment-month is a Lisp macro in `calendar.el'.

(calendar-increment-month MON YR N &optional NMONTHS)

Increment the variables MON and YR by N months.
Forward if N is positive or backward if N is negative.
A negative YR is interpreted as BC; -1 being 1 BC, and so on.
Optional NMONTHS is the number of months per year (default 12).