Many Twitter users found themselves unable to log in to the service on Sunday night, due to what appeared to be an authentication error — an issue that seemed to affect mostly users of the company’s Android app, but also reportedly affected some users of the website version, as well as the Windows app and some other third-party apps. After several hours worth of complaints, the Twitter status blog reported that there was a “login issue” and that engineers were working on it.
According to a number of reports, the authentication problem appeared to be caused by a date error on Twitter’s servers, which showed the current date as being December 28, 2015 — and this was supported by the fact that on the Tweetdeck app, current tweets were shown as having been posted 365 days ago. If there was a large date discrepancy with Twitter’s servers, many apps and services may have prevented users from logging in for security reasons.
Update: Twitter hasn’t confirmed what the cause of the authentication error was, but one theory making the rounds of the software engineering community is that the company’s servers were the victim of a common programming mistake involving the way that dates are displayed in code — in short, using the abbreviation “YYYY” instead of “yyyy” can cause a date-related mistake near the end of a year. A similar error affected Apple last year.