Fix time tracking in calendar helper, refs #19

Original Fix from Ben Zhang https://github.com/bzbnhang.
Original pull request https://github.com/teambox/teambox/pull/40

Original comment:
For instance, I have seted my time zone to Adelaide(9:30),
if I go to the calendar at 9:00AM morning the current day
is 8th Feb but in the Time tracking section, it shows today
is 7th Feb. This because 9AM in Adelaide is around 5PM in
San Francisco, which happens to be one day different.