Implementation for setTappedCityTimer: andButton: method in "Build a World Clock in Objective-C."

ViewController.m

    -(void)setTappedCityTimer { 
        [self.timer invalidate];
        self.timer = nil;

        [self setDateTimeLabelsWithTimeZone];
        self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(setDateTimeLabelsWithTimeZone) userInfo:nil repeats:YES];
    }