Time stamp

The most reliable way would be to use an Automation.  Create a field called [Current Date/Time] with a formula of 
Now()

Create another field called [Checkbox Date/Time]

The set up an Automation to trigger when the checkbox field changes and the checkbox ends up being checked to edit the record to copy the [Current Date/Time] into the [Checkbox Date/Time] .

You can also use a form rule, which is simpler, but if the form was sitting for a long time on the user's screen in edit mode, then the time stamp will be when the record was opened for edit and not when it  was saved.