Fix bug with parsing time formats during DST periods.

Fix bug with parsing time formats during DST periods.

wxDateTime::ParseFormat() used todays date as fall back when parsing all
formats, including those involving times, which meant that its results
depended on whether DST was active at the time of the parsing which was
clearly wrong.

Fix this by using a fixed date on which DST is known not to be active as fall
back date.

This fixes unit test failures in DateTimeTestCase::TestTimeFormat() when it
was ran on a DST transition date.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70993 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775