public void addEntry(View view) {
    final Dialog d = new Dialog(this);
    d.setContentView(R.layout.dialog);
    d.setTitle("Add Entry");
    d.setCancelable(true);
    d.show();

    ...

    chooseDate.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            String str = selectDate();
            date.setText(str);
        }
    });
}

public String selectDate(){

    final Dialog datePicker = new Dialog(this);
    datePicker.setContentView(R.layout.choose_date);
    datePicker.setTitle("Choose Date...");
    datePicker.setCancelable(true);
    datePicker.show();

    Button selectFinalDate = (Button) datePicker.findViewById(R.id.selectDate);
    final DatePicker dp = (DatePicker) datePicker.findViewById(R.id.datePicker1);

    selectFinalDate.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            strDateTime = (dp.getMonth() + 1) + "/" + dp.getDayOfMonth() + "/" + dp.getYear();
            datePicker.dismiss();
        }
    });
    return strDateTime;
};

...
 date.setText(str);
 view.invalidate(); // the view that you are showing in the dialog
...

/**
* global variable for your dialog view
*/
View view =null;

                // in your addEntry(View view)
                ...
                Dialog d = new Dialog(this);
                view  = LayoutInflater.from(this).inflate(R.layout.dialog, null);
                d.setContentView(view);
                ...

                 // selectDate()
                 ...
                 date.setText(str);
                 view.invalidate(); // the view that you are showing in the dialog
                 ...