treat two digit years less than 50 as 21st century years

One year ago, Mozilla changed the way Firefox interprets dates in the format mm/dd/yy.
In Firefox version 49 or later, if yy is less than 50, the year is regarded as 20yy;
if yy is 50~99, the year is regarded as 19yy. The old Firefox behavior matches IE and Edge,
and its new behavior matches Chrome and Safari.[1]

This PR changes the behavior of parsing two digit years to align with SpiderMonkey and v8.
Tests are included.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1265136

lib/Runtime/Library/DateImplementation.cpp

@@ -1464,6 +1464,10 @@ Error:
                 lwYear = -lwYear + 1;
             }
         }
+        else if (lwYear < 50 && isDateNegativeVersion5 == false)
+        {
+            lwYear += 2000;
+        }
         else if (lwYear < 100 && isDateNegativeVersion5 == false)
         {
             lwYear += 1900;

test/Date/TwoDigitYears.js

@@ -0,0 +1,41 @@
+//-------------------------------------------------------------------------------------------------------
+// Copyright (C) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE.txt file in the project root for full license information.
+//-------------------------------------------------------------------------------------------------------
+
+// A two digit year in the format mm/dd/yy is interpreted as a 21st century year if it is less than 50;
+// otherwise it's interpreted as a 20th century year.
+// See: https://github.com/Microsoft/ChakraCore/pull/4062
+
+// 21st century years:
+runTest("01/01/00", "2000-01-01T00:00:00");
+runTest("01/01/00 00:00:01 am", "2000-01-01T00:00:01");
+runTest("01/01/00 00:00:01 am EST", "2000-01-01T00:00:01-05:00");
+runTest("11/14/17", "2017-11-14T00:00:00");
+runTest("10/26/49", "2049-10-26T00:00:00");
+runTest("12/31/49 11:59:59 pm", "2049-12-31T23:59:59");
+runTest("12/31/49 11:59:59 pm PST", "2049-12-31T23:59:59-08:00");
+
+// 20st century years:
+runTest("01/01/50", "1950-01-01T00:00:00");
+runTest("01/01/50 01:34:59", "1950-01-01T01:34:59");
+runTest("09/27/70", "1970-09-27T00:00:00");
+runTest("12/31/99", "1999-12-31T00:00:00");
+runTest("12/31/99 11:59:59 p.m.", "1999-12-31T23:59:59");
+runTest("12/31/99 11:59:59 p.m. UTC", "1999-12-31T23:59:59Z");
+
+function runTest(dateToTest, isoDate) {
+    if (isoDate === null) {
+        if (isNaN(Date.parse(dateToTest))) {
+            console.log("PASS");
+        } else {
+            console.log("Wrong date parsing result: Date.parse(\"" + dateToTest + "\") should return NaN");
+        }
+    } else {
+        if (Date.parse(dateToTest) === Date.parse(isoDate)) {
+            console.log("PASS");
+        } else {
+            console.log("Wrong date parsing result: Date.parse(\"" + dateToTest + "\") should equal Date.parse(\"" + isoDate + "\")");
+        }
+    }
+}

test/Date/rlexe.xml

@@ -134,4 +134,9 @@
       <files>MilitaryTimeZone.js</files>
     </default>
   </test>
+  <test>
+    <default>
+      <files>TwoDigitYears.js</files>
+    </default>
+  </test>
 </regress-exe>