Calculate Textbox value based on another textbox value
 

I have a form where I load a date into Textbox1. I would like to forecast
Textbox2 to three weeks after Textbox1. What code can I use in Textbox 2 to
do that? What if my Textbox1 date is Jan 2, 2009 and I want to calculate in
Textbox2 to the next three week cycle date after say Mar 1, from Jan 3, 2009.

(i.e.) TextBox1 Value Jan 2 2009
Textbox2 Value (after Mar 3, 2009) Mar 6, 2009
Textbox3 Value Mar 27, 2009