Textbox (Working with Dates)

This is for a VBA TextBox (Not Worksheet).
I have two text boxes, both are going to have dates.
Textbox1 a user will enter a date; example 10/22/08 or 10-22-08 or 10-22-2008 etc, TextBox2 will get that date and add seven days.

How do I do that?
This obviously did not work.

TextBox8.Value = TextBox3.Value + 7