Convert NSTimeInterval to NSString in iPhone?

NSTimeInterval is just a double type so you can convert it to string using +stringWithFormat: method

NSTimeInterval  today = [[NSDate date] timeIntervalSince1970];
NSString *intervalString = [NSString stringWithFormat:@"%f", today];

-(NSString*)formattedDuration:(NSTimeInterval)interval{
    NSDateComponentsFormatter *formatter = [[NSDateComponentsFormatter alloc] init];
    formatter.allowedUnits = NSCalendarUnitHour | NSCalendarUnitMinute;
    formatter.zeroFormattingBehavior = NSDateComponentsFormatterZeroFormattingBehaviorPad;
    NSString *string = [formatter stringFromTimeInterval:interval];
    NSLog(@"%@", string);
    // output: 0:20:34
    return string;
}