> What's the history of human timekeeping? Particularly before the Gregorian calendar, what historical records do we have for who was tracking/tallying the days elapsed over time? How did people coordinate on the current date globally (if at all)? How did local mean time (LMT) work in the past?

Ooh, this is a really interesting topic!

Okay, so the first thing to keep in mind is that there are three very important cyclical processes that play a fundamental role in human timekeeping and have done so since well before anything we could detect archaeologically: the daily solar cycle, the lunar cycle (whence the month), and the solar year. All of these are measurable with mark 1 human eyeballs and nothing more technologically advanced than a marking stick.

For most of human history, the fundamental unit of time from which all other time units are defined is the day. Even in the SI system, a second wasn't redefined to something more fundamental than the Earth's kinematics until about 60 years ago. For several cultures, the daylight and the nighttime hours are subdivided into a fixed number of periods, which means that the length of the local equivalent of 'hour' varied depending on the day of the year.

Now calendars specifically refer to the systems for counting multiple days, and they break down into three main categories: lunar calendars, which look only at the lunar cycle and don't care about aligning with the solar year; lunisolar calendars, which insert leap months to keep the lunar cycle vaguely aligned with the solar year (since a year is about 12.5 lunations long); and solar calendars, which don't try to align the lunations (although you usually still end up with something akin to the approximate length of a lunation as subdivisions). Most calendars are actually lunisolar calendars, probably because lunations are relatively easy to calibrate (when you can go outside and see the first hint of a new moon, you start the new month) but one of the purposes of the calendar is to also keep track of seasons for planting, so some degree of solar alignment is necessary.

If you're following the history of the Western calendrical tradition, the antecedent of the Gregorian calendar is the Julian calendar, which was promulgated by Julius Caesar as an adaptation of the Egyptian solar calendar for the Romans, after a series of civil wars caused the officials to neglect the addition of requisite leap months. In a hilarious historical example of fencepost errors, the number of years between leap years was confused and his successor Augustus had to actually fix the calendar to have a leap year every 4th year instead of every third year, but small details. I should also point out that, while the Julian calendar found wide purchase in Christendom, that didn't mean that it was handled consistently: the day the year started varied from country to country, with some countries preferring Christmas as New Years' Day and others preferring as late as Easter itself, which isn't a fixed day every year. The standardization of January 1 as New Years' Day isn't really universal until countries start adopting the Gregorian calendar (the transition between Julian and Gregorian calendar is not smooth at all).

Counting years is even more diverse and, quite frankly, annoying. The most common year-numbering scheme is a regnal numbering: it's the 10th year of King Such-and-Such's reign. Putting together an absolute chronology in such a situation requires accurate lists of kings and such that is often lacking; there's essentially perennial conflicts in Ancient Near East studies over how to map those dates to ones we'd be more comfortable with. If you think that's too orderly, you could just name years after significant events (this is essentially how Winter Counts work in Native American cultures); the Roman consular system works on that basis. If you're lucky, sometimes people also had an absolute epoch-based year number, like modern people largely agree that it's the year 2025 (or Romans using 'AUC', dating the mythical founding of Rome), but this tends not to be the dominant mode of year numbering for most of recorded human history.