CalendarExtender Updating Two TextBox Objects

I am trying to figure out how to get the CalendarExtender or some other functionality that can operate with the CalendarExtender to update two textbox object. I need the CalendarExtender to update the textbox it is associated with (call it FromDate) and I need, when FromDate is set, for something to calculate the date of last day of the month that was selected in FromDate and place that date into another textbox (call it ToDate). I would like this to be done automatically without the need to clicking on a button or any other action by the user.

Can anyone point me in the correct direction?

Thanks, Randy