When you browser/OS has the timezone +12 (New Zealand)
and you goto to the self sign up form,
the UI tries to pre-select an appropriate timezone.
But in the case its +12 this does not work and the box stays empty (which will lead to that the user is created with the timezoneId 0, which leads to non working features).
1) The default selection should work
2) The sign up should throw an error if no timezone was selected. A user profile that has no timezone leads to errors and exceptions as the calculation in the calendar et cetera does no more work.