How do I get the number of days in the month specified by an elisp time?

Question

In elisp I have a time in the (form of three integers), and I can get the month using decode-time. What I'd like to get is the number of days in that month (and year), using elisp functions (rather than write my own).

i.e:

(defun days-in-month-at-time(t)
    ; Figure out month and year with decode-time
    ; return number of days in that month   
)

Answer 1

(require 'timezone)

(defun days-in-month-at-time (time)
  "Return number of days in month at TIME."
  (let ((datetime (decode-time time)))
    (timezone-last-day-of-month (nth 4 datetime) (nth 5 datetime))))

Answer 2

Looks like this is better since time-zone doesn't seem to be in all recent emacs versions edit: Sorry you just need to require timezone, or calendar depending on whether you use this or the other answer.

(defun days-in-month-at-time (time)
  "Return number of days in month at TIME."
  (let ((datetime (decode-time time)))
    (calendar-last-day-of-month (nth 4 datetime) (nth 5 datetime))))