According to When Did You Join Twitter? the Twitter epoch is March 21st, 2006, the date that founders @jack, @ev and @biz all first created their accounts.
I joined a few months later on July 18th, 2006. Find out when you signed-up.
Then there’s My First Tweet which helps you find your first post on Twitter.
Back then it was called Twttr until September 2006 when they added in the vowels.
