Fix date calculation if providing both a timestamp and a timezone.

We need to temporarily set the timezone to the one provided in the contructor,
because the result of getdate() depends on the current default timezone. It's
probably the only method in the whole world that does this.

Bug: 12843
Revert "Move tests to timezonesTest."

Bug is in Horde_Date.

This reverts commit 76a135f2560e0aded3b4cfbf9dbfa7e11ccd70eb.
Revert "Add currently failing test for Bug: 12843"

Bug is in Horde_Date.

This reverts commit 29ee3502dacc8fba07c7b355dbe60dbbf0e5e7d1.