While I have to know quite a few different time scales, I never really paid much attention to when these were developed: 1967 - the second was officially defined as 9,192,631,770 cycles of radiation due to transition between the two hyperfine levels in the ground state of the isotope cesium 133. I guess this is the date of our modern definition of time. 1972 - The new definition of time didn't correlate very well to the Earth's motion, so a new compromise time scale using the official second was developed: Coordinated Universal Time (UTC). This adds a leap second every so often to sync up UTC to the Earth's motion (There's a problem with this. Sometimes it's done after 12 months; sometimes it's done after 18 months. Timing is so critical with modern computer networks, no one wants to depend on humans to all manually adjust their clocks by one second when they're supposed to, so we haven't added a leap second since Jan 1, 1999. If we don't fix this urgent problem within a few thousand years, we'll be going to work when the sun sets and getting home when the sun rises). This, worst of all: Ephemeris Time was established in 1960. It went obsolete in 1984, being replaced by Terrestrial Dynamic Time, which was developed in 1977. Time scales have been born and died during my lifetime. AAAUGHHH! :surprised I could add other time scales added during the last 30 years, but I think Chicago's on the radio, "Does anybody really know what time it is? Does anybody really care?" On a less disturbing note, anybody know when the longest solar day of this year will be?