--- a/toolkit/mozapps/extensions/test/xpcshell/test_install.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_install.js
@@ -1,17 +1,18 @@
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
 // This verifies that add-ons can be installed from XPI files
 
-// The maximum allowable time since install. If an add-on claims to have been
-// installed longer ago than this the the test will fail.
-const MAX_INSTALL_TIME = 10000;
+// Maximum error in file modification times. Some file systems don't store
+// modification times exactly. As long as we are closer than this then it
+// still passes.
+const MAX_TIME_DIFFERENCE = 3000;
 
 Components.utils.import("resource://gre/modules/Services.jsm");
 Components.utils.import("resource://gre/modules/NetUtil.jsm");
 
 // install.rdf size, icon.png, icon64.png size
 const ADDON1_SIZE = 705 + 16 + 16;
 
 do_load_httpd_js();
@@ -133,16 +134,22 @@ function check_test_1() {
           zipReader.close();
         }
       }
       else {
         let iconFile = uri.QueryInterface(AM_Ci.nsIFileURL).file;
         do_check_true(iconFile.exists());
       }
 
+      // Make the pending install have a sensible date
+      let updateDate = Date.now();
+      let extURI = pendingAddons[0].getResourceURI("");
+      let ext = extURI.QueryInterface(AM_Ci.nsIFileURL).file;
+      setExtensionModifiedTime(ext, updateDate);
+
       // The pending add-on cannot be disabled or enabled.
       do_check_false(hasFlag(pendingAddons[0].permissions, AddonManager.PERM_CAN_ENABLE));
       do_check_false(hasFlag(pendingAddons[0].permissions, AddonManager.PERM_CAN_DISABLE));
 
       restartManager();
 
       AddonManager.getAllInstalls(function(activeInstalls) {
         do_check_eq(activeInstalls, 0);
@@ -154,24 +161,23 @@ function check_test_1() {
           do_check_eq(a1.name, "Test 1");
           do_check_true(isExtensionInAddonsList(profileDir, a1.id));
           do_check_true(do_get_addon("test_install1").exists());
           do_check_in_crash_annotation(a1.id, a1.version);
           do_check_eq(a1.size, ADDON1_SIZE);
 
           do_check_eq(a1.sourceURI.spec,
                       Services.io.newFileURI(do_get_addon("test_install1")).spec);
-          // Should have been installed sometime in the last two second.
-          let difference = Date.now() - a1.installDate.getTime();
-          if (difference > MAX_INSTALL_TIME)
-            do_throw("Add-on was installed " + difference + "ms ago");
-          if (difference < 0)
-            do_throw("Add-on was installed " + difference + "ms in the future");
+          let difference = a1.installDate.getTime() - updateDate;
+          if (Math.abs(difference) > MAX_TIME_DIFFERENCE)
+            do_throw("Add-on install time was out by " + difference + "ms");
 
-          do_check_eq(a1.installDate.getTime(), a1.updateDate.getTime());
+          difference = a1.updateDate.getTime() - updateDate;
+          if (Math.abs(difference) > MAX_TIME_DIFFERENCE)
+            do_throw("Add-on update time was out by " + difference + "ms");
 
           do_check_true(a1.hasResource("install.rdf"));
           do_check_false(a1.hasResource("foo.bar"));
 
           let uri = do_get_addon_root_uri(profileDir, "addon1@tests.mozilla.org");
           do_check_eq(a1.getResourceURI("install.rdf").spec, uri + "install.rdf");
           do_check_eq(a1.iconURL, uri + "icon.png");
           do_check_eq(a1.icon64URL, uri + "icon64.png");
@@ -244,17 +250,23 @@ function run_test_3(install) {
     ]
   }, [
     "onInstallStarted",
     "onInstallEnded",
   ], check_test_3);
   install.install();
 }
 
-function check_test_3() {
+function check_test_3(aInstall) {
+  // Make the pending install have a sensible date
+  let updateDate = Date.now();
+  let extURI = aInstall.addon.getResourceURI("");
+  let ext = extURI.QueryInterface(AM_Ci.nsIFileURL).file;
+  setExtensionModifiedTime(ext, updateDate);
+
   ensure_test_completed();
   AddonManager.getAddonByID("addon2@tests.mozilla.org", function(olda2) {
     do_check_eq(olda2, null);
     restartManager();
 
     AddonManager.getAllInstalls(function(installs) {
       do_check_eq(installs, 0);
 
@@ -264,24 +276,24 @@ function check_test_3() {
         do_check_eq(a2.version, "2.0");
         do_check_eq(a2.name, "Real Test 2");
         do_check_true(isExtensionInAddonsList(profileDir, a2.id));
         do_check_true(do_get_addon("test_install2_1").exists());
         do_check_in_crash_annotation(a2.id, a2.version);
         do_check_eq(a2.sourceURI.spec,
                     "http://localhost:4444/addons/test_install2_1.xpi");
 
-        // Should have been installed sometime in the last two second.
-        let difference = Date.now() - a2.installDate.getTime();
-        if (difference > MAX_INSTALL_TIME)
-          do_throw("Add-on was installed " + difference + "ms ago");
-        if (difference < 0)
-          do_throw("Add-on was installed " + difference + "ms in the future");
+        let difference = a2.installDate.getTime() - updateDate;
+        if (Math.abs(difference) > MAX_TIME_DIFFERENCE)
+          do_throw("Add-on install time was out by " + difference + "ms");
 
-        do_check_eq(a2.installDate.getTime(), a2.updateDate.getTime());
+        difference = a2.updateDate.getTime() - updateDate;
+        if (Math.abs(difference) > MAX_TIME_DIFFERENCE)
+          do_throw("Add-on update time was out by " + difference + "ms");
+
         gInstallDate = a2.installDate.getTime();
 
         run_test_4();
       });
     });
   });
 }