I am using the jQuery date picker calendar in a form. Once submitted the form passes params along via the url to a third party site. Everything works fine, except for one thing. If the value inserted into the date field by the datepicker calendar is subsequently deleted, or if the default date, that is in the form on page load, is deleted and the form is submitted I get the following error:

"Conversion from string "" to type 'Date' is not valid."

The solution to my problem is really simple, I want to validate the text field where the date is submitted and send out a default date (current date) if the field is empty for any reason. The problem is I am terrible at Javascript and cannot figure out how to do this.

Here is the form code for my date field.

[var('default_date' = date)]
<input type="text" id="datepicker" name="txtdate" value="[$default_date]" onfocus="if (this.value == '[$default_date]') this.value = '';" onchange="form.BeginDate.value = this.value; form.EndDate.value = this.value;" />
			<input type="hidden" name="BeginDate" value="[$default_date]"/>
			<input type="hidden" name="EndDate" value="[$default_date]"/>