It's not just calendars. It's clocks too. GMT is less than 200 YEARS old. Yet even more than that, GMT reflects recent changes to our very concept of time.<p>GMT was based on noon at the Royal Observatory. But only nominally because GMT was a standard for mechanized time and hence abstracted away the messy variability in Solar noon. The fact that you probably are not going to flame me for describing solar noon as 'messily' variable indicates how deeply anachronistic the table is with respect to the meaning of time.<p>To carry this further, the calendar is a mechanical abstraction over the messy details of years. The year for Stonehenge users is based on observable natural phenomena just as noon is when the sun is due south. And if your first thought wasn't to flame me for leaving half the world out of the description of noon that's your conceptual blindspot not mine.