For comparison, I developed a quick example for Android and iOS. In both apps, we add a clock that refreshes.

First in swift for ios. We need to add a UILabel in the view and name it timeLabel and create two files:


// in ViewController.swift

import UIKit

class ViewController: UIViewController {
    
    // preparing interval to update clock
    var timer: NSTimer?
    
    // link UILabel with controller 
    @IBOutlet weak var timeLabel: UILabel!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // create interval and define callback
        timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: #selector(ViewController.updateTimeLabel), userInfo: nil, repeats: true)
        
    }
    
    // on app exit clear interval
    deinit {
        timer?.invalidate()
    }
   
    // callback function 
    func updateTimeLabel() {
       // create a date format
        let formatter = NSDateFormatter()
        formatter.timeStyle = .MediumStyle
        // update label
        timeLabel.text = "\(formatter.stringFromDate(NSDate()))"
    }


}

In Android Studio it is a little bit easier. There is a TextClock object that we will add in the view. We link this element into our controller and update the date format.


// in MainActivity 

package com.example.patrick.clock;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextClock;


public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // get TextClock
        TextClock clock = (TextClock) findViewById(R.id.textClock);
        // set format
        clock.setFormat12Hour("hh:mm:ss");
    }

}