Can only enter datepicker once

I have a form with a date field in it but can only enter the datepicker once. As soon as I select a date from the datepicker, I get an "Error on page" message appearing in the lower left corner of the browser. If I double click on the error message icon message reads:

Message: Object doesn't support this property or method
Line: 70
Char: 3
Code: 0
URI: add_inv_create_items.php

If I view the scource of the webpage line 70 is in the following javascript function:

function closeDatePicker_Add_Invoice_group_Invoice_Date() {
var tElm = $('#Add_Invoice_group_Invoice_Date');
if (typeof Add_Invoice_group_Invoice_Date_Spry != null && typeof Add_Invoice_group_Invoice_Date_Spry != "undefined") {
Add_Invoice_group_Invoice_Date_Spry.validate();
}
var docElm = document.getElementById("Add_Invoice_group_Invoice_Date");
var tBlur = docElm.getAttribute("onBlur");
if (!tBlur) tBlur = docElm.getAttribute("onblur");
if (!tBlur) tBlur = docElm.getAttribute("ONBLUR");
if (tBlur) {
tBlur = tBlur.replace(/\bthis\b/g, "docElm");
eval(tBlur);
}
}

Line 70 is this line: tBlur = tBlur.replace(/\bthis\b/g, "docElm");