Convert RFC 3339 formatted date string to NSDate

gistfile1.m

//From https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/DataFormatting/Articles/dfDateFormatting10_4.html

- (NSDate *)dateFromRFC3339String:(NSString *)dateString {
    
    static NSDateFormatter *sRFC3339DateFormatter = nil;
    if (sRFC3339DateFormatter == nil) {
        sRFC3339DateFormatter = [[NSDateFormatter alloc] init];
        NSLocale *enUSPOSIXLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];
        
        [sRFC3339DateFormatter setLocale:enUSPOSIXLocale];
        [sRFC3339DateFormatter setDateFormat:@"yyyy'-'MM'-'dd'T'HH':'mm':'ss'Z'"];
        [sRFC3339DateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
    }
    
    // Convert the RFC 3339 date time string to an NSDate.
    NSDate *date = [sRFC3339DateFormatter dateFromString:dateString];
    
	return date;
}

Useful for In-App purchase StoreKit receipt validation