How do you write tests for methods that interact with data in system files?

I'm trying to be proactive about writing an iOS app in a test-driven manner. However, I'm stumped as to how to test a method that is to interact with real system files. For those familiar with iOS, I'm writing to the NSUserDefaults file.

Here's my set-up: I have a singleton called AppState that I am using to record when data downloaded from the Internet was last synced locally. Other classes can update the dateLastSynced property of AppState. I am overriding the setDateLastSynced method to save that update to the system so it's persisted between opening and closing the app:

- (void)setDateLastSynced:(NSDate *)dateLastSynced
{
    // update instance variable with new value
    _dateLastSynced = dateLastSynced;

    // store change to NSUserDefaults
    [[NSUserDefaults standardUserDefaults] setObject:dateLastSynced forKey:REFUGE_APP_STATE_DATE_LAST_SYNCED_KEY];
    [[NSUserDefaults standardUserDefaults] synchronize];
}

I'd like to write a test that verifies that setDateLastSynced is working correctly - but I'm not sure how to do that without overwriting data already saved to the system in NSUserDefaults.