How do i capture the time of the submit button for a calculation?

I'd like to calculate the time the user hits the submit button to perform a calculation.

EG, the form has a Start Time that the user enters manually.  I want the time the user fills out the form to be the end time.  And I want to have another calculated field that calculates the difference between the two times...

Thanks, but unfortunately that won't work because the start time is not the time they start entering the form...