There are varying messages that greet you upon the failure to log in, including “Something is technically wrong. … Thanks for noticing–we’re going to fix it up and have things back to normal soon.” as well as “Twitter is currently down for maintenance.” if you happen to be in select time zones, such as those who are in Eastern European Time.
Thankfully, Twitter is more than aware of the current issue, and are working hard to solve whatever problem that plagues them. Of course, speculations have been flying around, with some thinking that it happens to be a problem with the dates, since Tweetdeck is showing posts backdated by 365 days. Of course, for those who are still signed in, the service ought to be able to run fine and dandy, but select time zones do claim that it is under maintenance. Which group are you in?