Script working in IE not FF

[janetb]

Not a JS guru, so can someone tell me why the following works on a .net aspx page in Javascript for IE, but no FF?

            getAge = function(form) {
                document.getElementById('txtDOB').value = ((document.getElementById('txtDOB').value).replace(new RegExp("/", "g"), "-"));
                var now = new Date();
                var DOB = new Date(document.getElementById('txtDOB').value);
                var oneDay = 1000 * 60 * 60 * 24 * 365; //converts from milliseconds
                document.getElementById('txtAge').value = parseInt((now.getTime() - DOB.getTime()) / oneDay);
            }


<asp:TextBox ID="txtDOB" runat="server" onkeyup="javascript:getAge(this.form);" onkeydown="javascript:getAge(this.form);" />