diff --git a/lisp/image/gravatar.el b/lisp/image/gravatar.el
index f6f056a2ba..ebae356e26 100644
--- a/lisp/image/gravatar.el
+++ b/lisp/image/gravatar.el
@@ -51,6 +51,9 @@ gravatar-cache-ttl
   :group 'gravatar)
 (make-obsolete-variable 'gravatar-cache-ttl nil "28.1")

+(defconst gravatar-cache-expiry (* 12 60 60)
+  "Time to live for gravatar cache entries (12 hours.)")
+
 (defcustom gravatar-rating "g"
   "Most explicit Gravatar rating level to allow.
 Some gravatars are rated according to how suitable they are for
@@ -287,8 +290,7 @@ gravatar-retrieve
 (defun gravatar--prune-cache ()
   (let ((expired nil)
         (time (- (time-convert (current-time) 'integer)
-                 ;; Twelve hours.
-                 (* 12 60 60))))
+                 gravatar-cache-expiry)))
     (maphash (lambda (key val)
                (when (< (car val) time)
                  (push key expired)))