Revert change to EquivalentTime in date.js. The checks that I've removed in the last changed were essential after all.

Review URL: http://codereview.chromium.org/660375

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3999 ce2b1a6d-e550-0410-aec6-3dcde31c8c00