The phone loses the Time Zone setting.
Go to Settings->date & time, turn Set Automatic off, is your time zone box empty?
If so, this is the fix, if not, sorry:
Set the Time Zone to something other than what it should be or was to this point (e.g. if Eastern, select Central...). Close the option and see the wrong time? Good. Now open it back up and set the Time Zone to the one you should use and had before. Is it in the box? Good. If not, then you probably selected the same Time Zone as before and the box simply won't allow that one to be used until it is changed to another one and changed back.
If this doesn't work, then just change your time zone to a different one and change it back.
Sometimes simpler instructions work better.