Asked a similar question a few months back, about how accurate GPS based devices were for measuring the distance run on a track, was deleted as off topic. Anyway I had a search and play, appears a few bodies had already composed a few tables giving the found accuracy for various devices e.g GPS Accuracy of Garmin, Polar, and other Running Watches
This lead me to having a play with a Sony Smartwatch 3, Adidas miCoach speed Cell + x-cell combo, Polar H7 HRM and Watch combo, Runtastic Combo.... (was given a toy box a couple of months back to play with / evaluate). If you ignore the pairing with a phone being a bit temperamental the Adidas combination appeared fairly accurate, but you'll only be able to get at your data once in front of a PC. The others are really only suited for distance activities e.g. to stop an activity on the smartwatch will take a tap to activate the screen, a swipe to go from a time / distance display, to a screen with a pause button, a tap to pause, another swipe and tap to stop..... essentially stopping the timer will take a second or two, so not suitable for a sprint. Also found the error in the distance recorded, by any of the GPS devices based devices, was more dependent on the app you've chosen to use than the device itself, appears the app's have different sample rates eg. Google's My Tracks only captures your position every 10 secs (can be altered), and the GPS position is only accurate to 10m.
Anyway per @Darko Sarovic answer, I'd probably stick with a stopwatch, but if you have time to play look at creating your own home grown timing gate eg. Laser sensor project, or using a Lego light sensor eg. Lego Mindstorms Drag Race with Light Gate (photo finish), or one of the Raspberry Pi / arduino projects, or possibly play with a Sony Smartwatch; in theory it could be useful in combination with a phone mounted on a tripod at the finish line. As it can be used as a manual / noise activated trigger for the video camera on the phone. So as long as you're recording with a timestamp you'll have an accurate time. The watch has both Bluetooth and Wifi, so if out of Bluetooth range the Wifi connectivity may be good for a whole track, will have to have a play.