Twitter has experienced a partial outage but it isn't immediately clear what went wrong, the company said.
"Something is technically wrong," the microblogging service's welcome page said. "Thanks for noticing - we're going to fix it up and have things back to normal soon."
Users reported problems on the Twitter application for Android and iPhone smartphones, while tweets on the computer application TweetDeck were showing up as having been posted a year ago.
Twitter claimed 284 million active users at the end of September.
Sony earlier this month was hit by a sophisticated hacking attack that stole massive amounts of data from its servers.
The US has blamed North Korea for the attack, with the reclusive state furious at the release of Sony film The Interview, which parodies leader Kim Jong-Un.