1 January is not the date of Christ's birth, but the feast of the circumcision.
Caesar's estimate of the year at 365/ days was remarkably accurate, but it was still eleven minutes too long.
By the sixteenth century the error had accumulated to ten days.
When Constantine Christianised the Roman Empire in the fourth century AD, the Roman calendar became the Christian calendar.
It was adopted as such in AD 325, at the first General Council of the Christian church at Nicea, in what is now western Turkey.
The day, at least, is universally recognised on earth. The week of seven days is a purely human invention, with no equivalent in the motions of the heavens; its widespread use testifies only to the enormous influence on history of the ancient Babylonians, who invented it.
The month is roughly lunar, but only roughly; a lunar month is 29-and-a-half days, and there are twelve-and-a half lunar months in a year.Only the ignorant, they insist, will take any notice of the year 2000, since we shall still be in the twentieth century until it ends.The first millennium began on 1 January AD 1; the second millennium began on 1 January 1001; the third millennium will therefore begin on new year's day 2001.On one reasonable calculation, Christ was born in the year 1 BC, which means the millennium would indeed fall in the year 2000, on Christmas day.Measuring time The basic thing to understand about the calendar is that it is a human, not a natural, construction. We take our time from the movements of the heavens.The day is the commonest and most straightforward measure of time.