Display correct day of the week with negative UTC offset

A duplicate subtraction of the UTC offset and DST time caused the creation
of an incorrect timestamp. In return this caused us to go back too far and
therefore moved us back one day.

Conflicts:
	root/portal/modules/portal_calendar.php
Merge pull request #117 from marc1706/calendar/one_day_off

Display correct day of the week with negative UTC offset