File constructor understands lastModified.

LayoutTests/fast/harness/should-be-now.html

Index: LayoutTests/fast/harness/should-be-now.html
diff --git a/LayoutTests/fast/harness/should-be-now.html b/LayoutTests/fast/harness/should-be-now.html
index f60a549c425a2eb944505c5411b05d69d9a7cae1..036461348887840f68facad63f38802c49e0290f 100644
--- a/LayoutTests/fast/harness/should-be-now.html
+++ b/LayoutTests/fast/harness/should-be-now.html
@@ -31,6 +31,26 @@ stubDateNow(60000, function() {
     shouldBeNow("70000");
 });
 
+debug("Testing a slightly past date with the implicit delta. This should pass.");
+stubDateNow(60000, function() {
+    shouldBeNow("59990");
+});
+
+debug("Testing a slightly future date with the implicit delta. This should pass.");
+stubDateNow(60000, function() {
+    shouldBeNow("60010");
+});
+
+debug("Testing a past date with a large delta. This should pass.");
+stubDateNow(60000, function() {
+    shouldBeNow("50000", 10000);
+});
+
+debug("Testing a future date with a large delta. This should pass.");
+stubDateNow(60000, function() {
+    shouldBeNow("70000", 10000);
+});
+
 debug("Simulating a defective clock that always goes backwards. The test below should fail.");
 var badClock = Date.now();
 var realDateNow = Date.now;