When you select a time zone from the selectbox, the fields below it are reset to default values. The issue here is that there is a short ajax delay between the selection and the reset which causes the fields being reset right after they are filled out by selenium.
I tried using _wait_for_visible_element instead but that obviously didnt work. :-/
So there are two possible solutions I can think of at the moment:
  a) Set up some value for the date/time first, then select timezone and wait for the date/time to change, then continue (plus exception for when we dont change the timezone - just a note)
  b) time.sleep(1)