Set Ajax calendar date

I am using the Ajax Calendar (http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/Calendar/Calendar.aspx) tied to a textbox.

What i would like to have is to set the value to minus 20 years from the current year but still have the textbox showing a blank value (as i have a Required Field Validator set to the calendar textbox) so the user is forced to select a date but dont have to trawl through all the years in order to get to the date they need.

My attempts so far leaves the textbox with a value and wondered if anyone has any suggestions?

Thanks