What we want: 1. A second has fixed length. 2. At 12 noon every day the sun is at its zenith. 3a. A day has 86400 seconds. 3b. Time only goes forward. Pick an

What we want: 1. A second has fixed length. 2. At 12 noon every day the sun is at its zenith. 3a. A day has 86400 seconds. 3b. Time only goes forward. Pick any three. not 1; 2, 3a, 3b: You get UT1 [1], which basically gives you the angle of the earth wrt the sun. Time proceeds continuously with 86400 seconds a day, but they're of varying length (depending on the earth rotation's speed) to keep up with where the sun is. This is measured by the International Earth Rotation Service [2]. 1, not 2, 3a, 3b: You get TAI [3]. Time proceeds continuously in SI seconds with 86400 seconds a day, but 12 noon slowly goes out of sync with the earth rotation. 1, 2, not 3a, 3b: You get UTC. It's using SI seconds, tries to keep in sync with the earth's rotation, but therefore must occasionally include leap seconds, so there will sometimes be 86401 seconds in a day (the watch will display 23:59:59 for a second and then 23:59:60 for a second). 1, 2, 3a, not 3b: You get UNIX time [4]. It's using SI seconds, tries to keep in sync with the earth's rotation, but when a leap second occurs, it will "jump back" and repeat a second (the watch will display 23:59:59 for two seconds). [1] https://en.wikipedia.org/wiki/Universal_Time https://en.wikipedia.org/wiki/Universal_Time [2] https://en.wikipedia.org/wiki/International_Earth_Rotation_and_Reference_Systems_Service https://en.wikipedia.org/wiki/International_Earth_Rotation_a... [3] https://en.wikipedia.org/wiki/International_Atomic_Time https://en.wikipedia.org/wiki/International_Atomic_Time [4] https://en.wikipedia.org/wiki/Unix_time https://en.wikipedia.org/wiki/Unix_time