Doc holds up the stopwatches.
Light Emitting Diode stopwatches were used by Emmett Brown in his initial time travel experiment. They were both synchronized until one traveled one minute into the future, skipping over the minute that the other watch experienced.
Behind the scenes
Doc and Einstein wore matching stopwatches with liquid-crystal displays, except during the two scenes in which he compares the watches before and after the experiment. Apparently, the time on these stopwatches was too dark to read, so these two scenes were refilmed with LED stopwatches.