Bug 12669: Use floating timezone when handling dates without hours, minutes and seconds.

Bug 12669: Use floating timezone when handling dates without hours, minutes and seconds.

Test plan:

1. Create a new serial record or pick an existing one;
2. Add a "New subscription" and set the "First issue publication date"
   and "Frequency" values so that the "Planned date" for a given issue
   hits a day where a transition to Daylight Saving Time (DST) occurs

   ex:. 1979-04-01 in Europe/Lisbon (http://www.timeanddate.com/time/change/portugal/lisbon?year=1979)
   ... use this website page as a source http://www.timeanddate.com/time/dst/2014.html
   to find a suitable transition to DST in your timezone in the current year.

3. From the "Subscription detail" page in Koha for the given record click
   the "Serial collection" link on the left side context menu.

Expected result:
Prior to applying the patch the error described in this bug report should occur.
After the patch is applied the "Serial collection" page should load correctly.

Signed-off-by: Colin Campbell <colin.campbell@ptfs-europe.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit e8f9000fcca4fd542f7f82e2288d3a627053dd77)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>