[_time]. You can use them in your Message body and when some one sends the form you get the message with date and time.
Go to your Message Body in the Mail tab, and add this code and save the changes.
From: [your-name] <[your-email]> Subject: [your-subject] Message Body: [your-message] Send on: [_date], [_time] -- This e-mail was sent from a contact form on WordPress Playground (http://localhost/wp-playground)
When someone fill in the form and send the message, you will get something like this.