My idea was to use two clocks, your wrist watch and an "atomic clock". You can nowadays buy an "atomic clock" for $50 almost everywhere. These clocks are not really atomic, but they receive a signal from atomic clocks, and automatically set themselves. Most are accurate to half second. Since atomic clocks are, if they are, corrected behind the scenes, the "atomic clock" you bought will be "updated" too, while your wrist watch goes it's own tempo. The difference between both clocks will grow over the years, I guess, and this is what you should measure.
Offered by Kiko.
Atomic clocks are rather unwieldy and fragile. They work by electrifying a vapor like strontium, cesium, or krypton; then sensing instruments detect the vibrations given off by the vapor, these vibrations are counted and the count is converted into time units like seconds and hours. Atomic clocks are very stable time references because the vibrations are produced at the atomic level and do not change because the properties of the atoms do not change.
Offered by Steve.