How to add pop-up calendar in backend of Joomla

Step 1: Add to header JHTML::_( 'behavior.calendar' );

<?php
defined( '_JEXEC' ) or die( 'Restricted access' ); 
JHTML::_( 'behavior.calendar' );  // <-- this line
?>

Step 2: Add your input field and image, with correct ids 

<input name="entrydate" id="entrydate" type="text" /> 
<img class="calendar" 
src="/templates/system/images/calendar.png" 
alt="calendar" id="entrydate_img" / >

Step 3: Add Mootools javascript code

<script  type="text/javascript">
window.addEvent('domready', function() {Calendar.setup({
        inputField     :    "entrydate",     // id of the input field
        ifFormat       :    "%Y-%m-%d",      // format of the input field
        button         :    "entrydate_img",  // trigger for the calendar (button ID)
        align          :    "Bl",           // alignment (defaults to "Bl" = Bottom Left, 
// "Tl" = Top Left, "Br" = Bottom Right, "Bl" = Botton Left)
        singleClick    :    true
    });});
</script>