I need help building a pendulum or another kind of time-keeping device?

Alright so for a school project, I need to build a time-keeping device acurate to .1 of a second. I was just planning on building a pendulum and using my eyes to count/guesstimate the time elapsed based on the number of swings. Does anyone have any better ideas? If not, can someone guide me through the process of building a pendulum? I was just planning on getting some scrap wood and building a hangman style contraption to which I would attach some string with a weight at the bottom of it and using trial and error to get it to swing and have the duration of the swing last a second by using a stopwatch to time it...any better suggestions?