add date that form was submitted to form results

There is actually a submissions date on every form submissions you receive which you can find on your form submissions page. 

Example:

You can also add a {DATE} variable on your email notification. This will be replaced with an actual date when you receive a submission base on your profile timezone.

Here's an article about using dates on email http://www.jotform.me/help/68-Using-Dates-in-Email-Templates.

If you have further questions, let us know.