1st millennium
From Wikinfo
In the Christian calendar, the 1st millennium usually refers to the period AD1-1000 inclusive. The year 1000 forms part of it, as otherwise it would have only 999 years. Similarly, the second millennium was AD1001-2000, including the year 2000. Thus the Millennium celebrations on 31 December 1999 were premature.
Strictly, the above should be called the 1st millennium AD. Sometimes, especially in archaeology, the 1st millennium is the period 1000-1BC (the 1st millennium BC).
