Age verification in Objective-C

Question

I'm creating some age-verification functionality for an iOS app. The user is presented with a UIDatePicker object, and the latest selectable date should be today minus 18 years. How vulnerable to inaccuracy is my code? How could it be leaner?

-(void)validateAge {

    NSDateComponents *today = [[NSCalendar currentCalendar] components:NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit fromDate:[NSDate date]];
    NSInteger day = [today day];
    NSInteger month = [today month];
    NSInteger year = [today year];

    int correctYear = year - 18;

    NSDateComponents *correctAge = [[NSDateComponents alloc] init];
    [correctAge setDay:day];
    [correctAge setMonth:month];
    [correctAge setYear:correctYear];

    NSCalendar *calendar = [NSCalendar autoupdatingCurrentCalendar];

    UIDatePicker *agePicker = [[UIDatePicker alloc] initWithFrame:CGRectMake(0, 240, 320, 200)];
    [agePicker setDatePickerMode:UIDatePickerModeDate];
    [agePicker setMaximumDate:[calendar dateFromComponents:correctAge]];
    [self.view addSubview:agePicker];
    return;
}

Answer 1

Check this method. Pass value as minus 18 (-18)

-(NSDate *)offsetYear:(int)numOfYears date:(NSDate*)date {
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDateComponents *offsetComponents = [[NSDateComponents alloc] init];
    [offsetComponents setYear:numOfYears];

    return [gregorian dateByAddingComponents:offsetComponents
                                  toDate:date options:0];
}

Answer 2

One basic idea about human-computer interaction is that a user should always be able to select something on the interface and go forward somewhere. You don't want to just leave them 'on hold'.

In your implementation, if the user is under 18, the interface will force them to lie about their birth date.

A much better idea would be to allow the user to select any date, but compute the difference in years to perform different operations depending upon the difference. You do this using

NSDateComponents *components = [calendar components:NSYearCalendarUnit
                                           fromDate:enteredDate
                                             toDate:[Date new] // today
                                            options:0];

After that, for people under 18, you can e.g. display an alert and redirect them to a different app.

It would still be possible for users to lie about their birth dates, of course, but the interface will not encourage it as much.