Fix generic wxTimePickerCtrl on DST change dates.

Fix generic wxTimePickerCtrl on DST change dates.

Do not use the date when DST changes as the date part of wxDateTime object
used by wxTimePickerGeneric internally as this introduces problems with times
that can't be represented at all -- and so can't be entered into the control.
Use arbitrary date on which DST does not change to avoid this.

Closes #14137.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71005 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775