JavaScript value from pop-up window

This seems a bit trivial but it is not working for me.

I have a form with a text field and an icon next to it that, when clicked, launches a pop-up window for selecting a date from a calendar.  The calendar package is the Tigra Calendar javascript module.

After selecting a date the text field is populated with the date in yyyy-mm-dd format.

In some other javascript code, I would like to check that a date is chosen.  I simply want to check if the field has a value or not.  But for some reason, when I check this field in the code, it does not find the element value.  I set an alert(scan_date) but no alerts pop-up after choosing the date.  If I manually type in the date value in the field, it does get this value and the alert pops up.

For getting the value, I use this line:

var scan_date = document.getElementById("scan_date").value;

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The PHP code is:

<input type=text id="scan_date" name="scan_date" value="<?php echo $scan_date;?>" class=input maxlength=10> <?php echo $flag;?>
<a href="javascript:cal1.popup();"><img src="img/cal.gif" width="16" height="16" border="0" alt="Click here to pick the date"></a>

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After the form closing tag I have:

var cal1 = new calendar1(document.forms['addnew'].elements['scan_date']);
cal1.year_scroll = true;
cal1.time_comp = false;

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I figured that's all the information that I need to put here for the experts to figure this out.  It seems that it should be pretty straightforward.