Stop user from selecting past date and time in datepickerdialog and timepickerdialog android

I am developing my first location reminder app where I add tasks with locations and date and time, for which when I reach I have to be notified. In the add task operation, I have the task name, location, buttons for the date and time (when the DatePickerDialog and TimePickerDialog buttons are clicked) and the AddTask button. I want the user not to select the past date and time from the dialog. How can i do this??

public class AddTasks extends Activity {

TaskForm activityForm = new TaskForm();

private Button pPickDate;
private int pYear;
private int pMonth;
private int pDay;
public String date;
public String Loc;
public double lat;
public double lon;

private Button pPickTime;
private int mHour;
private int mMinute;
public String time;

private TimePickerDialog timePickerDialog;
private DatePickerDialog datePickerDialog;

Date selectedDate;


/** This integer will uniquely define the dialog to be used for displaying date picker.*/
static final int DATE_DIALOG_ID = 0;
static final int TIME_DIALOG_ID = 1;

/** Callback received when the user "picks" a date in the dialog */
private DatePickerDialog.OnDateSetListener pDateSetListener =
        new DatePickerDialog.OnDateSetListener() {

            public void onDateSet(DatePicker view, int year,
                                  int monthOfYear, int dayOfMonth) {
                pYear = year;
                pMonth = monthOfYear;
                pDay = dayOfMonth;
               updateDate();
                displayDateToast();
            }
        };

private TimePickerDialog.OnTimeSetListener mTimeListener =
        new TimePickerDialog.OnTimeSetListener() {
            public void onTimeSet(TimePicker view, int hour, int minute) {
                mHour = hour;
                mMinute = minute;
                updateTime();
                displayTimeToast();
            }
        };

/** Updates the date in the TextView */
private void updateDate() {
  /**  pDisplayDate.setText( */ date =
            new StringBuilder()
                    // Month is 0 based so add 1
                    .append(pDay).append("/")
                    .append(pMonth + 1).append("/")
                    .append(pYear).append(" ").toString();
   // date = pDisplayDate.getText().toString();

}

private void updateTime() {
   /** pDisplayTime.setText( */ time =
            new StringBuilder()
                    .append(mHour).append(":")
                    .append(mMinute).append("").toString();
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.add_task);
    init();

    /** Capture our View elements */
    //pDisplayDate = (TextView) findViewById(R.id.displayDate);
    pPickDate = (Button) findViewById(R.id.pickDate);

    /** Listener for click event of the button */
    pPickDate.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            showDialog(DATE_DIALOG_ID);
        }
    });

   /** Capture our View elements */
    //pDisplayTime = (TextView) findViewById(R.id.displayTime);
    pPickTime = (Button) findViewById(R.id.pickTime);


    /** Listener for click event of the button */
    pPickTime.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            showDialog(TIME_DIALOG_ID);
        }
    });

    /** Get the current date and time */
    final Calendar cal = Calendar.getInstance();
    pYear = cal.get(Calendar.YEAR);
    pMonth = cal.get(Calendar.MONTH);
     pDay = cal.get(Calendar.DAY_OF_MONTH);

    mHour = cal.get(Calendar.HOUR_OF_DAY);
    mMinute = cal.get(Calendar.MINUTE);
}

/** Create a new dialog for date and time picker */
@Override
protected Dialog onCreateDialog(int id) {
    switch (id) {
        case DATE_DIALOG_ID:
            datePickerDialog =  new DatePickerDialog(this,
                    pDateSetListener,
                    pYear, pMonth, pDay);
            return datePickerDialog;

        case TIME_DIALOG_ID:
            timePickerDialog = new TimePickerDialog(this,
                    mTimeListener,
                    mHour,mMinute,true) ;
             return timePickerDialog;
    }
    return null;
}

public void add_task(View view) {

    if(activityForm.getTaskNameInput().getText() != null &&
            activityForm.getTaskLocInput() != null && activityForm.getTaskDateInput() != null &&
            activityForm.getTaskTimeInput() != null) {

        Task task = new Task();
        task.setTaskNameString(activityForm.getTaskNameInput().getText()
                .toString());
        task.setTaskLocString(activityForm.getTaskLocInput());
        task.setTaskLat(activityForm.getTaskLat());
        task.setTaskLon(activityForm.getTaskLon());
        task.setDateString(activityForm.getTaskDateInput());
        task.setTimeString(activityForm.getTaskTimeInput());

        persistTask(task);

        Toast.makeText(getApplicationContext(), "SAVED=" + task.hashCode(),
                Toast.LENGTH_LONG).show();

        activityForm.getTaskNameInput().setText(" ");

        Calendar c = Calendar.getInstance();
        if (timePickerDialog != null) {
            timePickerDialog.updateTime(c.get(Calendar.HOUR_OF_DAY), c.get(Calendar.MINUTE));
        }

        if (datePickerDialog != null) {
            datePickerDialog.updateDate(c.get(Calendar.YEAR), c.get(Calendar.MONTH),
                    c.get(Calendar.DAY_OF_MONTH));
        }
    }

    else {
        AlertDialog.Builder dialog = new AlertDialog.Builder(this);
        dialog.setTitle("Attention");
        dialog.setMessage("Please ensure you are setting all the values");
        dialog.setPositiveButton("Ok", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int id) {

            }
        });
        dialog.show();
    }
}

class TaskForm  {
private EditText taskNameInput;
private String taskLocInput;
private Double taskLat;
private Double taskLon;
private String taskTimeInput;
private String taskDateInput;

/**
 * @return the taskNameInput
 */
public EditText getTaskNameInput() {
    return taskNameInput;
}

/**
 * @param taskNameInput
 *            the taskNameInput to set
 */
public void setTaskNameInput(EditText taskNameInput) {
    this.taskNameInput = taskNameInput;
}

/**
 * @return the taskLocInput
 */
public String getTaskLocInput() {
    return taskLocInput;
}
/**
 * @param taskLocInput
 *            the taskLocInput to set
 */
public void setTaskLocInput(String taskLocInput) {
    this.taskLocInput = taskLocInput;
}

public Double getTaskLat(){
    return taskLat;
}
public void setTaskLat(Double taskLat){
    this.taskLat = taskLat;
}
public Double getTaskLon(){
    return taskLon;
}
public void setTaskLon(Double taskLon){
    this.taskLon = taskLon;
}

/**
 * @return the taskDateInput
 */

public String getTaskDateInput(){
    return taskDateInput;
}

/**
 * @param taskDateInput
 *            the taskDateInput to set
 */

public void setTaskDateInput(String taskDateInput){
    this.taskDateInput = taskDateInput;
}

/**
 * @return the taskTimeInput
 */
public String getTaskTimeInput() {
    return taskTimeInput;
}

/**
 * @param taskTimeInput
 *            the taskTimeInput to set
 */
public void setTaskTimeInput(String taskTimeInput) {
    this.taskTimeInput = taskTimeInput;
}

      

        
        
        
      

    

}