Fix bug in parsing dates that fall in the DST "non existant hour"

Summary:
The old code was creating a datetime from maybeDate + maybeTime
but that meant that it could be creating a datetime that never
existed if that datetime falls in the time that jumps to the
future when doing dst (i.e 2->3 am)

Test Plan: Added new test that fails without my patch and works with it

Reviewers: mlaurent

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D10218