Don't do that, just use a timer call before and after the event, and it'll tell you exactly how long it took. At it's most simplistic, put the current time into a variable before you run the AppleScript, then put the time into a different variable when it's done. Calc the difference and display.
I took this simplicity to advanced levels of complexity in MultiTimer, but you can use it in simple mode, too—just call it with startquiet and then again with end.
-rob.