You are needlessly complicating things. Why are you converting the date to a time interval (then the time interval to a different primitive)? Just [sharedDefaults setObject:theDate forKey:@"theDateKey"] and be done with it. NSDate is one of the “main types” supported by the PLIST format (dates, numbers, strings, data, dictionaries, and arrays), so you can just store it directly.
See the documentation for proof.
Just store and retrieve the date directly and watch it Do The Right Thing (including time zones, precision, etc). There is no formatter involved, as others have said.