populating a txtbox

I have two txtboxs. Txtbox1 I let the user populate with a date then I want to populate txtbox2 with a date that is 7 days later. I'm using this code in my codebehind page to populate txtbox2, which is working fine.

txtbox2.Text = DateAdd(DateInterval.Day, 7, CDate(txtbox1.Text))

What I want to know, is there a way to have txtbox2 populate as soon as txtbox1 is filled without having to call the page, so the user knows they won't have to fill in the second date?

thanks,
eyeYou will have to use javascript to do it.