You can set a field's value to the now() function to determine the time a button was clicked. You can find some very complex formula samples online for determining the difference between two date / time fields - the math on those is outside of what I can understand, and I use code / qRules to determine date diffs - but it can be done with rules, apparently. If your form is submitted to a SharePoint library, it might be simpler to use a calculated field in the library for the diff, frankly. If you use submit rules, you can have the button click first set a field's value to the now() function and then run the rule to submit the form, followed by a rule to close the form.
Hilary Stoupa
