Over thousands of years humanity has devised various means of measuring time. Time expresses actual processes in the physical world. Some subjective measurements of time, for example, years and days, do relate approximately to natural frames of reference, the rotation of the earth around the sun or the earth's daily rotation on its axis. Other measurements, such as weeks, hours and minutes, represent practical attempts to measure change.

However, the accepted definition of a millennium, any period of one thousand years, did not originate from nature or practical calendrics. It does not correspond to factual astronomical cycles, unlike the primary cycles of days, lunations and years, or the practical needs of humanity, but rather social factors, the peculiarities of Christianity. It is the domain of eschatology that bears responsibility for the arbitrary construction of the millennium.

Millennial thinking is embedded in the apocalyptic writings of the bible. The traditional Christian millennium is the future reign of Jesus lasting one thousand years, following a final battle between Christ and Satan. Satan loses, and is cast into the lake of fire and brimstone, and Jesus wins, overseeing a Last Judgement of all the dead.

The failed consummation of the second coming resulted over the generations in ardent Christian followers setting, and then postponing, the date of the future apocalypse. But how did the apocalyptic millennium transform itself into a calendrical measurement, the completion of a secular period of a thousand years in human history? The answer is related to the failure of the expected biblical millennium to materialise.

To get from the apocalyptic to the calendrical millennium required a convoluted rummage through biblical symbolism and the tried and tested method of analogy. This is how it works: "One day is with the Lord as a thousand years, and a thousand years as one day" (2 Peter 3:8). The Book of Revelation informs us that the first post-apocalyptic age, the millennium, will endure for a thousand years. In God time that means a day. Genesis tells us that God created the world in six days and put his feet up on the seventh. In God time, using symbolic comparison, this means that world history unfolds for six thousand years, followed by a final thousand-year period of millennial heaven on earth. Put like that it makes sense, if you ignore the opening two chapters of the Book of Genesis which make it clear that the Creation had come and gone.

Just one small problem though: when exactly is the earthly six thousand year period up? In other words, when precisely is the millennium (the apocalyptic that is, not the calendrical)? Or are they the same?

Attention then focused on the year 2000. Modern historical scholarship in the 17th century demanded that in order to prove creation in 4000 BC symbolic analogy be corroborated with data from history. The idea was hit upon, using the bible and other historical documents, of counting back from the birth of Jesus, through the duration of Roman and Near Eastern empires, the reigns of the kings of Judah and Israel, the ages of the patriarchs, and last but not least, the week of creation, to see whether the newly acquired preferred date tallied with the historical record.

It was Archbishop James Ussher who performed this arduous task in 1650, setting the time of creation at noon on 23 October 4004 BC. The four-year discrepancy is explained by an error made by the sixth-century inventor of the BC-AD system, a monk by the name of Dionysius Exignus. Herod died in 4 BC, so, barring being compelled to delete some oft-told stories from the New Testament, Jesus could not have been born later than 4 BC, not 1 AD as commonly believed. We then logically count back four thousand years and conclude that the world was created in 4004 BC

This caused heated debates over the years on when exactly a millennium, or for that matter a century, ends. For Gould common-sense means with the years marked '99 yet, according to Gould, historical records show that New Year celebrations took place on January 1 1701, 1801 and even 1901. Gould explains how in the past scholars and those in power overwhelmingly favoured centuries beginning with years marked '01 rather than '00. Not wanting to undermine the authority of the church, they had to accept, unwittingly or not, the perceived mistaken calendrical calculations of Dionysius that Jesus was born in 1 AD, the Year of Our Lord.

In fact Dionysius calculated that Jesus was born eight days before 1st January 1 AD, on 25th December 1 BC. On a Christian basis therefore, if we ignore the correct date of Herod's death, 1st January 2000 is actually justified as honouring two thousand years since the nativity in 1 BC. Mathematically, though, there won't have been 2,000 AD years until the end of 31st December 2000.

Today most people accept the logic of centuries beginning in years marked '00, and so the new calendrical millennium is being celebrated on 1st January 2000, a nice round number, and just ninety-nine years since the beginning of the 20th century.