Longhorn D1 (A.K.A M9, or milestone 9) development concludes: November 10, 2004More info at Flexbeta
Longorn Beta 1 internal development: November 11, 2004 to November 24, 2004
Longhorn Beta 1 lockdown: November 29, 2004 to January 14, 2005
Longhorn Beta 1 escrow: January 17, 2005 to February 16, 2005
Longhorn Beta 1 release (client and Server): February 16, 2005
Longhorn to have reached Beta status?

Well folks, its taken a long time, but from what we know Longhorn has reached the beta stage. Internal development on Beta 1 started on November 11th."