[FIX] calendar: fix allday onchange user tz

While choosing a start/stop datetime in a timezone hour
were the UTC value is within the next day,
e.g.
Start time Dec 22 20:00 in US/Eastner (UTC -5:00)
Therefore, start time Dec 23 01:00 in UTC
Checking the "all day" box made the start time set to
Dec 23 instead of what the user expected, Dec 22.

This is because the onchange simply took the UTC datetime
(Dec 23 01:00) from which it removed the time (Dec 23).

This revision make sure to remove the time from the
user timezoned datetime instead, so the day set
remain the same day than what was set in the datetime.

opw-702065