Android - implement a custom timer/clock

I have an android app where user presses a start button and some function of collecting data is started. I have a custom EditText that shows the time that all this process has taken and is updated every second till user presses stop. I implemented this using an AsyncTask like this below:

    protected class RecTimer extends AsyncTask<Context, Integer, String> 
     {
                 @Override
                 protected String doInBackground( Context... params ) 
                 {
                          int sec=00;
                      int min=00;
                      int hours=00;
                      timerdata=timer.split(":");
                      hours=Integer.parseInt(timerdata[0]);
                      min=Integer.parseInt(timerdata[1]);
                      sec=Integer.parseInt(timerdata[2]);
 
                         while(Recording==true) 
                         {
                             try{
 
                                         Thread.sleep( 1000 );
                                         publishProgress( sec , min , hours );
                                         sec++;
 
                                         if(sec==60){
 
                                             min++;
                                             sec=0;
 
                                             if(min==60){
                                                 hours++;
                                                 min=0;
                                             }
                                         }
                                 } catch( Exception e ){
                                         Log.i("makemachine", e.getMessage() );
                                 }
                         }
                         return "COMPLETE!";
                 }
 
                 @Override
                 protected void onPreExecute() 
                 {
                         Log.i( "makemachine", "onPreExecute()" );
                         super.onPreExecute();                   
                 }
 
                 @Override
                 protected void onProgressUpdate(Integer... values) 
                 {
                         super.onProgressUpdate(values);
                         //Log.i( "makemachine", "onProgressUpdate(): " +  String.valueOf( values[0] ) );
 
                         String format = String.format("%%0%dd", 2);
                         String Sec = String.format(format, values[0]);
                         String Min = String.format(format, values[1]);
                         String Hours = String.format(format, values[2]);
 
                         ((EditText) findViewById(R.id.duration1)).setText(Hours + ":"+ Min + ":" + Sec);
 
                 }
 
 
                 @Override
                 protected void onPostExecute( String result ) 
                 {      
                         super.onPostExecute(result);
                         Log.i( "makemachine", "onPostExecute(): " + result );
 
                }          
         } 
 

It seemed to work fine at first, but now I see that if the app is idle and out of focus the timer may not always count well. I mean that I leave for example the device for 15 minutes and then see that in my "clock" only 5 have passed. Since the view is back on focus everything seems to work normally again.

First of all why does this happen? Shouldn't the AsyncTask be always executed in background? Is there some other better approach I should use to implement this?

Thanks to everybody in advance!

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