Date field format based on digital signature date

I have a form with a digital signature field, which includes the following execution field when the field is signed:

this.getField("Date").value = "Digital Signature: " + util.printd("yyyymmdd", new Date());

The corresponding "Date" field has the format set as yyyymmdd to match above.

However, upon digitally signing the document I get the following Warning: JacaScript Window -

Invalid date/time: please ensure that the date/time exists. Field [Date] should match format yyyymmdd

The "Date" field is formatted to yyyymmdd, and it even displays a correct example of the result on the format tab, so I don't understand why the error message comes up.