The short answer is you can't.
One thing to realize is the signature appearance is not the signature. The signature itself is a blob of data written into the PDF file. What you see on the page is a pictorial representation of the signature. One of the items in the signature proper is the date which is neither American format (month/day/year) nor is it European format (day/month/year), but rather Generalized Time format which takes values of the year, month, day, hour, time, minute, and second. All you can do is either display the date or not.
However, if you want to get creative you could put the date into either the Location or Reasons field and turn of the use of Labels. Just a suggestions.
I also thought so. I needed something as proof to let my managers know.