What's the optimum way of storing an NSDate in NSUserDefaults?

Question

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.

For option #1, I don't believe a date formatter is involved. Possibly under the hood, but I imagine it's not broken. The date is stored in ISO 8601 form.

For option #2, use -setDouble:forKey: and -doubleForKey instead of the float-based versions. That might be the cause of your precision errors.

Use NSUserDefaults; dates are stored in Zulu time, so there are no time zone issues to worry about. Store it in your time zone, pull it out in another time zone, you'll be fine, the system handles the conversion (no date formatter to worry about).

If you are saving the expiration date from the Facebook Graph API, I would use *Option 2*.

Option two can be easily converted to a string (using stringWithFormat). Most importantly, it works for the Graph API.

Plus, you don't have to worry about the format of your date. Not dealing with NSDateFormatter is worth the possibility of a 48 second error.