Modal pop up calendar control issue

I am using a Calendar extender in a modalpopup 开发者_运维问答when i click the image button the calendar extender should be displayed and when i click a particular date that date should be displayed in the textbox its working but what my problem is when i click the image button for the first time the calendar extender is displayed in the back side of the modal pop hence i am unable to select a particular date. but when i click the same image button for the second time the calendar extender will be displayed correctly.

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Add following code in head section of the page

<style type="text/css"> 
   .ajax__calendar_container { z-index : 1000; } 

</style> 

Note:Please change the value of z-index accordingly.

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//use java script
function calendarShown(sender, args)
{
    sender._popupBehavior._element.style.zIndex = 10005;
}