The booth timer is used by interpreters to keep track of their shifts. See more information here http://interstaricp.com/forums/topic/booth-timer/
If your booth timer starts with the delay or adds a few seconds, it means your computer clock is not synchronized.
Open Settings on Windows 10.
Click on Time & Language.
Click on Date & time.
Under the “Synchronize your clock” section, click the Sync now button.