I’m having the same problem. All my single day events now show as having an end date, and that end date shows as being the day before the start date!
If I delete the end date and save then it comes back again.
For the record the date/time settings in my WordPress install are:
Timezone: Edmonton (UTC time is 2013-12-19 18:44:07 Local time is 2013-12-19 11:44:07)
Date Format: December 19, 2013
Time Format: 11:44 am
Week starts on: Monday
