--- a/dom/alarm/AlarmService.jsm
+++ b/dom/alarm/AlarmService.jsm
@@ -326,26 +326,33 @@ this.AlarmService = {
       }.bind(this),
       function getAllErrorCb(aErrorMsg) {
         throw Components.results.NS_ERROR_NOT_IMPLEMENTED;
       }
     );
   },
 
   _getAlarmTime: function _getAlarmTime(aAlarm) {
-    let alarmTime = (new Date(aAlarm.date)).getTime();
+    // Avoid casting a Date object to a Date again to
+    // preserve milliseconds. See bug 810973.
+    let alarmTime;
+    if (aAlarm.date instanceof Date) {
+      alarmTime = aAlarm.date.getTime();
+    } else {
+      alarmTime = (new Date(aAlarm.date)).getTime();
+    }
 
     // For an alarm specified with "ignoreTimezone", it must be fired respect
     // to the user's timezone.  Supposing an alarm was set at 7:00pm at Tokyo,
     // it must be gone off at 7:00pm respect to Paris' local time when the user
     // is located at Paris.  We can adjust the alarm UTC time by calculating
     // the difference of the orginal timezone and the current timezone.
-    if (aAlarm.ignoreTimezone)
+    if (aAlarm.ignoreTimezone) {
        alarmTime += (this._currentTimezoneOffset - aAlarm.timezoneOffset) * 60000;
-
+    }
     return alarmTime;
   },
 
   _sortAlarmByTimeStamps: function _sortAlarmByTimeStamps(aAlarm1, aAlarm2) {
     return this._getAlarmTime(aAlarm1) - this._getAlarmTime(aAlarm2);
   },
 
   _debugCurrentAlarm: function _debugCurrentAlarm() {

--- a/hal/tests/browser_alarms.js
+++ b/hal/tests/browser_alarms.js
@@ -9,17 +9,17 @@ XPCOMUtils.defineLazyModuleGetter(this, 
  * not officially allow a integer, but nor does it disallow it. Of course this
  * test will break if AlarmService adds a type check, hence this note.
  * FIXME: when bug 810973 is fixed.
  */
 
 function add_alarm_future(cb) {
   let alarmId = undefined;
   AlarmService.add({
-    date: Date.now() + 143,
+    date: new Date(Date.now() + 143),
     ignoreTimezone: true
   },
   function onAlarmFired(aAlarm) {
     ok(alarmId === aAlarm.id, "Future alarm fired successfully.");
     cb();
   },
   function onSuccess(aAlarmId) {
     alarmId = aAlarmId;
@@ -29,17 +29,17 @@ function add_alarm_future(cb) {
     cb();
   });
 }
 
 function add_alarm_present(cb) {
   let self = this;
   let alarmId = undefined;
   AlarmService.add({
-    date: Date.now(),
+    date: new Date(),
     ignoreTimezone: true
   },
   function onAlarmFired(aAlarm) {
     ok(alarmId === aAlarm.id, "Present alarm fired successfully.");
     cb();
   },
   function onSuccess(aAlarmId) {
     alarmId = aAlarmId;
@@ -48,17 +48,17 @@ function add_alarm_present(cb) {
     cb();
   });
 }
 
 function add_alarm_past(cb) {
   let self = this;
   let alarmId = undefined;
   AlarmService.add({
-    date: Date.now() - 5,
+    date: new Date(Date.now() - 5),
     ignoreTimezone: true
   },
   function onAlarmFired(aAlarm) {
     ok(alarmId === aAlarm.id, "Past alarm fired successfully.");
     cb();
   },
   function onSuccess(aAlarmId) {
     alarmId = aAlarmId;
@@ -67,17 +67,17 @@ function add_alarm_past(cb) {
     ok(false, "Unexpected error adding alarm for time in the past " + error);
     cb();
   });
 }
 
 function trigger_all_alarms(cb) {
   let n = 10;
   let counter = 0;
-  let date = Date.now() + 57;
+  let date = new Date(Date.now() + 57);
   function onAlarmFired() {
     counter++;
     info("trigger_all_alarms count " + counter);
     if (counter == n) {
       ok(true, "All " + n + " alarms set to a particular time fired.");
       cb();
     }
   }
@@ -89,17 +89,17 @@ function trigger_all_alarms(cb) {
         ignoreTimezone: true
       },
       onAlarmFired
     );
   }
 }
 
 function multiple_handlers(cb) {
-  let d = Date.now() + 100;
+  let d = new Date(Date.now() + 100);
   let called = 0;
 
   function done() {
     if (called == 2) {
       ok(true, "Two alarms for the same time fired.");
       cb();
     }
   }
@@ -127,17 +127,17 @@ function multiple_handlers(cb) {
       ignoreTimezone: true
     },
     handler2
   );
 }
 
 function same_time_alarms(cb) {
   var fired = 0;
-  var delay = Date.now() + 100;
+  var delay = new Date(Date.now() + 100);
 
   function check() {
     fired++;
     if (fired == 4) {
       ok(true, "All alarms set for the same time fired.");
       cb();
     }
   }