More about the measurement of time...
Last time, we reached 1761, when the first marine chronometer allowed the accurate reconciliation of a local time with the time at a fixed meridian, enabling mariners to calculate longitude. The base meridian chosen ran through the Royal Observatory at Greenwich, England; thus came the time standard we know as GMT, or Greenwich Mean Time. GMT was, and is, used to uniquely identify an epoch, the moment when an instantaneous event occurs, without confusing references to local time zones. The military calls GMT Zulu Time, and uses it to coordinate worldwide drills, alerts, and, I guess, the odd war. Actually, the designation GMT has been largely superseded, circa 1928, by Universal Time. Same standard, different name.
To be thorough, I should now briefly describe the following standard designations of epoch, based variously on manifestations of gravitation, electromagnetism, rotational inertia and radioactivity: atomic time, dynamical time, Ephemeris Time, sidereal time, solar time, standard time, and, of course, Coordinated Universal Time.
However, I see by the millisecond hand on my cesium chronometer that this subject has worn out its welcome. Instead, letís talk about broccoli!