Account for timezone when creating test date

The timezone-offset for a given user may result in the expression "new Date(0)" returning a date that, when stringified, will result in a time before 1970. To work around this, factor in the user's timezone-offset when instantiating the test date.
Merge pull request #423 from laser/bugs/failing-date-test

BUG: toString Date-test fails when run with negative timezone-offset