Bug 9031: Overdue items crossing DST boundary throw invalid local time exception

Bug 9031: Overdue items crossing DST boundary throw invalid local time exception

To test:
1 - Set TZ to America/New York
2 - Checkout item and set due date to '2016-03-09 02:29:00"
3 - Make sure fines are set for the item type, fine mode production,
  calculate fines on return
4 - Check in item - invalid date time warning in logs
5 - Apply patch
6 - Check in item - no error
7 - prove t/Calendar.t

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Bug 9031: Use floating instead of UTC

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Squashed the changes for Calendar.pm; will add a follow-up to finally
overcoming the crash on Invalid local time.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>