How to do it without looking at my timer?
in earlier versions of Dynamo where tuneup isnt available. then you can use a definition which forces a recalculation of the date time at the point it is triggered.
The simplest and out of the box solution (that works with most Dynamo versions) I can think of is to add file creating nodes as the first and the last node in your script. You can compare timestamp on both files to get exact exeuction time.