Fix time tests by adding Date to sandbox.

Since tests are now run in their own sandbox, assert.deepEqual was not
properly testing the returned Date objects for equality, as they weren't
instances of the same Date class used by the test itself, causing type
inference to fail.  It was always returning true, even for different
dates.
Merge pull request #1236 from mbostock/time-sandbox

Fix time tests by adding Date to sandbox.