MainActivity

on start button click do:

 Calendar calendar = Calendar.getInstance();
        calendar.set(Calendar.HOUR_OF_DAY, 7);
        calendar.set(Calendar.MINUTE, 0);
        calendar.set(Calendar.SECOND, 0); 
        Intent intent = new Intent(getApplicationContext(), myService.class);
        intent.setAction("start");
        PendingIntent pi = PendingIntent.getService(getApplicationContext(), 0,
                intent, PendingIntent.FLAG_NO_CREATE);
        AlarmManager am = (AlarmManager) getApplicationContext().getSystemService(Context.ALARM_SERVICE);
        am.setRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(),
                AlarmManager.INTERVAL_DAY, pi);



on stop button click do:


        Calendar calendar = Calendar.getInstance();
        calendar.set(Calendar.HOUR_OF_DAY, 12);
        calendar.set(Calendar.MINUTE, 0);
        calendar.set(Calendar.SECOND, 0); 
        Intent intent = new Intent(getApplicationContext(), myService.class);
        intent.setAction("stop");
        PendingIntent pi = PendingIntent.getService(getApplicationContext(), 0,
                intent, PendingIntent.FLAG_NO_CREATE);
        AlarmManager am = (AlarmManager) getApplicationContext().getSystemService(Context.ALARM_SERVICE);
        am.setRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(),
                AlarmManager.INTERVAL_DAY, pi);