iOS get seconds by string

NSString *datestr = @"2013/05/02 08:06";

NSDateFormatter *df = [[NSDateFormatter alloc] init];
[df setDateFormat:@"yyyy/MM/dd H:mm"];
    
NSDate *ticket_date = [df dateFromString:datestr];
NSTimeInterval seconds = [ticket_date timeIntervalSinceNow];
    
NSLog(@"%@  seconds : %f", datestr, seconds);