If you need four-digit years, set dateformat:using the exact format you want. The downside is that you lose the automatic formatting of the locale.
NSDateFormatter *dateFormatter = [[[NSDateFormatter alloc] init] autorelease];
[dateFormatter setDateFormat:@"MM/dd/yyyy"];
...
birthday.text = [dateFormatter stringFromDate:p.Birth];
If you need localization, you can try changing the format of the short style date.
NSDateFormatter *dateFormatter = [[[NSDateFormatter alloc] init] autorelease ];
[dateFormatter setDateStyle:NSDateFormatterShortStyle];
[dateFormatter setTimeStyle:NSDateFormatterNoStyle];
if ([[dateFormatter dateFormat] rangeOfString:@"yyyy"].location == NSNotFound) {
NSString *fourDigitYearFormat = [[dateFormatter dateFormat] stringByReplacingOccurrencesOfString:@"yy" withString:@"yyyy"];
[dateFormatter setDateFormat:fourDigitYearFormat];
}
Updated with Bugfix from Max Macleod