diff --git a/emcclib.js b/emcclib.js
index 84336f3..497451a 100644
--- a/emcclib.js
+++ b/emcclib.js
@@ -194,29 +194,37 @@ mergeInto(LibraryManager.library, {
     /*
      * void js_activate_timer();
      *
-     * Start calling the C timer_callback() function every 20ms.
+     * Start calling the C timer_callback() function every frame.
      */
     js_activate_timer: function() {
-        if (timer === null) {
-            timer_reference_date = (new Date()).valueOf();
-            timer = setInterval(function() {
-                var now = (new Date()).valueOf();
-                timer_callback((now - timer_reference_date) / 1000.0);
-                timer_reference_date = now;
-                return true;
-            }, 20);
+        if (!timer_active) {
+            timer_reference = performance.now();
+            var frame = function(now) {
+                current_timer = null;
+                timer_callback((now - timer_reference) / 1000.0);
+                /* The callback may have deactivated the timer. */
+                if (timer_active) {
+                    timer_reference = now;
+                    current_timer = window.requestAnimationFrame(frame);
+                }
+            }
+            timer_active = true;
+            current_timer = window.requestAnimationFrame(frame);
         }
     },
 
     /*
      * void js_deactivate_timer();
      *
-     * Stop calling the C timer_callback() function every 20ms.
+     * Stop calling the C timer_callback() function every frame.
      */
     js_deactivate_timer: function() {
-        if (timer !== null) {
-            clearInterval(timer);
-            timer = null;
+        if (timer_active) {
+            timer_active = false;
+            if (current_timer !== null) {
+                window.cancelAnimationFrame(current_timer);
+                current_timer = null;
+            }
         }
     },
 
diff --git a/emccpre.js b/emccpre.js
index 22c462b..f9f0c18 100644
--- a/emccpre.js
+++ b/emccpre.js
@@ -90,8 +90,8 @@ var midpoint_test_str = "ABCDEFGHIKLMNOPRSTUVWXYZ0123456789";
 var midpoint_cache = [];
 
 // Variables used by js_activate_timer() and js_deactivate_timer().
-var timer = null;
-var timer_reference_date;
+var timer_active = false;
+var timer_reference;
 
 // void timer_callback(double tplus);
 //