As of February 20th, 2013, you’re going to have two fewer characters to send a message on Twitter when you’re sharing a link. The t.co wrapper that Twitter is using to shorten web addresses is lengthening from 20 to 22 reserved characters. This means that any link you share will now take up two more spaces that you could previously use for letters.
The HTTP links will go from 20 to 22 and HTTPS (secure) links will go from 21 to 23 characters reserved for the t.co short-linking wrapper on Twitter. If you’re unfamiliar with the way this works, when you share a link on Twitter, it is automatically shortened, and takes up exactly 20 characters, leaving you 120 to tweet with. Now, you’ll have 118 (or 117 if it’s a secure link).
So, when you’re sharing tweets with links after that date, you’re going to have to slice them down by two more characters. Tweets without links will be unaffected – so no worries there.