Android : Alarm between a specific interval of time

I am new in android. I am working on an android alarm app.I want to show an alarm between 8am to 8pm after each 30 minutes and repeat this alarm daily. My code is here. Main Activity....

 import java.util.Calendar;

 import android.app.Activity;
 import android.app.AlarmManager;
 import android.app.PendingIntent;
 import android.content.Context;
 import android.content.Intent;
 import android.os.Bundle;
 import android.view.View;
 import android.widget.Button;
 import android.widget.CompoundButton;
 import android.widget.Switch;
 import android.widget.Toast;

 public class WaterActivity extends Activity {

Button leftBTN, rightBTN;
Switch toggleSwitch;
Context context = this;
SharedPref pref;
static PendingIntent pendingIntent;
static AlarmManager alarmManager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_water);
    leftBTN         =   (Button)findViewById(R.id.leftbtn);
    rightBTN        =   (Button)findViewById(R.id.rightbtn);
    toggleSwitch    =   (Switch)findViewById(R.id.switchbtn);
    pref            =   new SharedPref(context);
    if(pref.getValue(pref.getWaterKey()).equals("on"))
        toggleSwitch.setChecked(true);

    toggleSwitch.setOnCheckedChangeListener(SwitchListener);
    leftBTN.setOnClickListener(new Button.OnClickListener() {
        @Override
        public void onClick(View v) {
            leftBTN.setTextColor(getResources().getColor(R.color.water_color));
            rightBTN.setTextColor(getResources().getColor(R.color.white_color));
            leftBTN.setBackgroundResource(R.drawable.button_bg);
            rightBTN.setBackgroundResource(R.drawable.button_right_bg2);


        }
          });
    rightBTN.setOnClickListener(new Button.OnClickListener() {
        @Override
        public void onClick(View v) {
            rightBTN.setTextColor(getResources().getColor(R.color.water_color));
            leftBTN.setTextColor(getResources().getColor(R.color.white_color));
            rightBTN.setBackgroundResource(R.drawable.button_right_bg);
            leftBTN.setBackgroundResource(R.drawable.button_bg2);


        }
          });
}
private Switch.OnCheckedChangeListener SwitchListener        =   new Switch.OnCheckedChangeListener(){

    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
         if (isChecked) {
             pref.savePre_value(pref.getWaterKey(), "on");
             Toast.makeText(context, "Reminder is on", Toast.LENGTH_SHORT).show();
                Intent intentsOpen = new Intent(context, WaterReceiver.class);
                pendingIntent = PendingIntent.getBroadcast(context,111, intentsOpen, 0);
                alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
                Calendar calendar = Calendar.getInstance();
                calendar.setTimeInMillis(System.currentTimeMillis());
                alarmManager.setRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), 10000, pendingIntent);

            } else {
                 pref.savePre_value(pref.getWaterKey(), "off");
                 Toast.makeText(context, "Reminder is off", Toast.LENGTH_SHORT).show();
                 alarmManager.cancel(pendingIntent);
            }
        }
};
@Override
public void onBackPressed() {
    // TODO Auto-generated method stub
    super.onBackPressed();
    finish();
}
}

and this is my Receiver.

  public class WaterReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
    Toast.makeText(context, "Time is up!!!!.",
            Toast.LENGTH_LONG).show();

}


 }

Please help what am i missing here? How can i do this in android.any one can help me in this? Thanks in start.