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.
The sun didn't change. The way people date the sun did.
A Julian calendar prior to October 1582 and from there on a Gregorian calendar<p><a href="http://en.wikipedia.org/wiki/Gregorian_calendar" rel="nofollow">http://en.wikipedia.org/wiki/Gregorian_calendar</a>
This posting made me look back to another posting on HN some time ago about 'Falsehoods Programmers believe about Time'.<p>My HN search fu is not great, it took me a while to find the posting so I post it here for the convenience of others: <a href="https://news.ycombinator.com/item?id=4128208" rel="nofollow">https://news.ycombinator.com/item?id=4128208</a>
1581 AD 10 Mar 18:01 11 Jun 19:34 13 Sep 06:54 11 Dec 20:06 00:02<p>1582 AD 10 Mar 23:56 12 Jun 01:30 13 Sep 12:39 22 Dec 01:54 00:02<p>1583 AD 21 Mar 05:51 22 Jun 07:16 23 Sep 18:24 22 Dec 07:42 00:02<p>Looks the single most dramatic shift happened in these two years. Would someone with astronomical chops care to explain a bit?