'But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up.'Click for footnote
The end of this world will come unexpected, 'like a thief in the night'. Such - very clear - statements can be found throughout the Bible, prominently also in the Synoptic Apocalypses. No one but God will know the exact date. How can man calculate something that only God knows? Augustine of Hippo lived and taught this knowledge, for example in his City of God, speaking of the ages of the world: 'The sixth is now passing, and cannot be measured by any number of generations, as it has been said, "It is not for you to know the times, which the Father hath put in His own power."'Click for footnote

But when the direct fulfilmentClick for footnote of the Second Coming of Christ failed, people began to calculate nonetheless, and continue to do so to this day. The passing of such a calculated date without the world ending has not led to any slackening of the belief in the actual fulfilment of the prophecy, though. '...those who fear feel relief at the passing of a date; those who hope feel disappointment and redate.'Click for footnote The failure of any set date is due to a mistake in calculation - on the human side. This does not question the truthfulness of divine revelation.

A popular basis for such calculations was the pattern of the sabbatical millenniumClick for footnote, which also formed the core of the idea of the 'aetates mundi'. The calculations on the ending of the sixth age of the world lead to several deadlines for apocalyptic expectations, such as 500 AD and 800 AD. The switch from the Annus Mundi to Anno Domini then led to the dates 1000 AD and 1033 AD, a thousand years after the passion of Christ.

Even though millenarian activity existed at these dates (Click for examples), these beliefs were not commonly shared by everyone. Belief that the end of the world would come with the new millennium was shared by some influencial personalities, but it was not all-embracing.
The Thief in the Night Signs of Doom Anglo-Saxon Expectations