Is it a leap year

Here's a bit of elisp code to find out...

(defun is-century-year(year)
  (if (= (mod year 100) 0)
      t
    nil))

(defun is-leap-year(year)
  "leap year is century years divisible by 400, 
not other century years, otherwise anything evenly 
divisble by 4"
  (if (is-century-year year) 
      (if (= 0 (mod year 400))
   t
 nil)
    (if (= 0 (mod year 4))
 t
      nil)))