Linked here is a the first twitter tweet by Jack Dorsey on March 21, 2006 at 1:02PM.
This information came from a tweet by Jack linked here on March 13, 2011 which says …
“5 years ago today we started programming Twitter (“twttr” for short). 8 days later the first tweet was sent: http://t.co/Vi5ii5A #twttr”