File constructor understands lastModified.

LayoutTests/resources/js-test.js

 // svg/dynamic-updates tests set enablePixelTesting=true, as we want to dump text + pixel results
 if (self.testRunner) {
     if (self.enablePixelTesting)
         testRunner.dumpAsTextWithPixelResults();
     else
         testRunner.dumpAsText();
 }
 
 var description, debug, successfullyParsed;
 
@@ skipping 551 matching lines @@
     if (typeof _e == "undefined" || _exception == _ev)
       testPassed(_a + " threw exception " + _exception + ".");
     else
       testFailed(_a + " should throw " + (typeof _e == "undefined" ? "an exception" : _ev) + ". Threw exception " + _exception + ".");
   } else if (typeof _av == "undefined")
     testFailed(_a + " should throw " + (typeof _e == "undefined" ? "an exception" : _ev) + ". Was undefined.");
   else
     testFailed(_a + " should throw " + (typeof _e == "undefined" ? "an exception" : _ev) + ". Was " + _av + ".");
 }
 
-function shouldBeNow(a)
+function shouldBeNow(a, delta)
 {
+    // Right now, V8 and Chromium / Blink use two different clock
+    // implementations. On Windows, the implementations are non-trivial and can
+    // be slightly out of sync. The delta is intended to compensate for that.
+    //
+    // FIXME: reconsider this when the V8 and Blink clocks get unified, see http://crbug.com/324110
+    if (delta === undefined)
+        delta = 1000;
+
     for (var i = 0; i < 1000; ++i) {
         var startDate = Date.now();
         var av = eval(a);
         var date = av.valueOf();
         var endDate = Date.now();
 
         // On some occasions such as NTP updates, the current time can go
         // backwards. This should only happen rarely, so we can get away with
         // retrying the test a few times if we detect the time going backwards.
         if (startDate > endDate)
             continue;
 
         if (typeof date !== "number") {
             testFailed(a + " is not a number or a Date. Got " + av);
             return;
         }
-        if (date < startDate) {
+        if (date < startDate - delta) {
             testFailed(a + " is not the curent time. Got " + av + " which is " + (startDate - date) / 1000 + " seconds in the past.");
             return;
         }
-        if (date > endDate) {
+        if (date > endDate + delta) {
             testFailed(a + " is not the current time. Got " + av + " which is " + (date - endDate) / 1000 + " seconds in the future.");
             return;
         }
 
         testPassed(a + " is equivalent to Date.now().");
         return;
     }
     testFailed(a + " cannot be tested against the current time. The clock is going backwards too often.");
 }
 
@@ skipping 142 matching lines @@
     testPassed = function(msg) {
         workerPort.postMessage('PASS:' + msg);
     };
     finishJSTest = function() {
         workerPort.postMessage('DONE:');
     };
     debug = function(msg) {
         workerPort.postMessage(msg);
     };
 }