Fixed a bug in cloning a Date

To clone a Date object d you need to convert it to a Number first and then pass
it to Date constructor, like new Date(+d). Otherwise some browsers (firefox)
cut milliseconds.
Fixed wrong test message

60 * 60 * 1000 is 3,600,000, not 360,000
Merge pull request #629 from ichernev/fix-test-diff

Fixed a bug in cloning a Date