onkeyup event is not working if i enter data from calendar control in textbox

Hi friends,
               i have a two textboxes with calendar controls. now i want rasie onkeyup event for textboxes to check whether two textboxes having text or not . if i enter data into textboxes with keyboard onkeyup event working very well.if i enter data from calendar control onkeyup event for textboxes not raising here is my code please help me to raise onkeyup event even if i select date from calendar control aslo

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script language="javascript" type="text/javascript">


 function SetButtonStatus(sender, target)
 {
 debugger
    var first = document.getElementById( ...