How to add days in a date

In: 02/11/13 --->DatetimePicker
Days: 10 ---->Textbox
Out: 02/21/13

How can I add the number of days in IN DATE that the answer will be automatic in the OUT DATE?
w/out using a button, it just when you type the number DAYS TEXTBOX it will auto add to the OUT DATE.

OutDateTimePicker.value = InDateTimePicker.Value.AddDays(txtdays.text) is this correct?