By any chance, are you trying to work with a date that is in the past?

If so, try changing the end time BEFORE setting the date.

This is an unfortunate consequence of these factors all coming together:
1. Choosing to stop a timer for a date/time from a previous day
2. Our decision to enforce setting of an end time to a date and time after the start time
3. The behavior of Apple's time and date pickers
4. The particular behavior of those pickers for people who use a 12 hour time format

Fortunately, it is easy enough to work around. You just have to set the end time before you can set the date.