[Almost] every country on Earth uses the ISO 8601 Gregorian Calendar for civil timekeeping—and with good reason: it is unquestionably the most-accurate calendar yet invented—perhaps the most-accurate it’s possible even to define.
Hmm, well I very much doubt that most countries have legislated that years must be -/0/+ rather than BC/AD.
And there are at least two calendars that are more accurate than the Gregorian: Herschel’s proposal to skip leaps in Gregorian years 4000 x N; and the reformed Julian calendar used by some orthodox christian churches. There are some nicely brief and informative articles at timeanddate.com though they are not good at distinguishing observational and arithmetical calendars.
Upon further reading, I recommend stumbling upon the Int’l Astronomical Union’s Standards of Fundamental Astronomy software algorithms for C/F77.
For example, their 67pp guide to SOFA Time Scale and Calendar Tools is quite good.
Hmm, well I very much doubt that most countries have legislated that years must be -/0/+ rather than BC/AD.
And there are at least two calendars that are more accurate than the Gregorian: Herschel’s proposal to skip leaps in Gregorian years 4000 x N; and the reformed Julian calendar used by some orthodox christian churches. There are some nicely brief and informative articles at timeanddate.com though they are not good at distinguishing observational and arithmetical calendars.