Thank you for the post.
While there are a number of ways to do this, one way would be to use 3 Fields: TimeStart (Time Field), TimeStop (Time Field), and TotalTime (Calculation Field). Two buttons could be used, START and STOP. The START button sets a variable using the Get (CurrentTime) function and then the "Set Field" script step to set the field TimeStart to the result. The STOP button sets a variable using the Get (CurrentTime) function and the the "Set Field" script step to set the field TimeStop to the result. The third field TotalTime is a calculation field with the calculation TimeStop - TimeStart.
Please note that you are posting to the Report an Issue part of the forum. It is intended for posting possible bug reports. Had you posted this to the FM Pro Forum section (see tab at top of screen), your post would have been seen by more people and probably gotten a quicker response.