How to reproduce:
1. Create a new event
2. Set an "Event Start" to a specific date and time such as: 05/05/2012 19:30
3. Set an "Event end" to a specific date and time such as: 05/05/2012 22:00
4. Click the "More Actions" and select "Publish/Unpublish on Specific Date"
5. Leave the "Publication date" on the default value
6. Change the "Unpublish on" value to a specific date and time such as: 05/12/2012 00:00
7. Click "Done" and "Save"
Note that the event list shows now shows "Published, scheduled unpublish on 12/05/12 at 00:00
" for this item (see create_event.png). As an aside, note that the date format is incorrect for my locale (should be 05/12/2012). As another aside, also note that the time formatting is incorrect for my locale--times are shown in 24hr format (should be AM/PM).
Now, click the newly added item and make a simple edit to the event text and click the "Publish" button.
Note that the event list shows now shows "Published
" and that the unpublish date and time have been wiped out (see edit_event.png).