The first, still existing, Twitter account was registered on March 21st, 2006 at 20:50 (UTC time), and every other Twitter account has been registered between that moment and now. Here you can find out for any Twitter account exactly how long it has been registered on Twitter, and for what percentage of the existence of Twitter it has been registered.

For the calculations of hours and days spent sending tweets, it was assumed that on average it takes 30 seconds to send a tweet.