The social network celebrated 2015 in style, by breaking its Android app and mobile website – and all, it seems, because of one misplaced letter

If you’re worried about how your New Year’s Eve will go, don’t. It’s not even 2015 yet, and Twitter’s already had a worse one than you.

The service was down for many users over five and a half hours on Monday morning UK time, between midnight and 5am (7pm to midnight ET, and 4pm to 9pm PT), after a bug in a line of code caused the service to think that it was 29 December, 2015.

Ninji the [REDACTED] (@_Ninji) I MITMed Twitter for Android's login to see why it was failing. The Twitter servers think it's 2015. Amazing. pic.twitter.com/iEu4rEUub9

That bug caused users of the network’s Android app and mobile website to be logged out, without any ability to log back in until it was fixed at 5.25am. It also led a number of visual flaws, such as showing TweetDeck users that tweets were posted 365 days ago.

tef (@tef) a single character G instead of Y took down twitter's api because the ISO year starts today — thursday is in january. welcome to computers.

How did it happen? Well, while today may be in the dying days of 2014, according to international standards, this week is the first of 2015. When a piece of software asks which year it is, the developer has to specify what they actually mean: do they mean the year that we are in today, or the year that this week is in?

Facebook Twitter Pinterest The difference one letter makes… Photograph: screenshot

Each of those is represented by just one character in many coding languages, and the wrong one comes first alphabetically. Assuming that this is the error that brought Twitter down overnight, it’s easy to see how it happened.

If the outage was caused by the date bug, it will also put fears to rest that Twitter had followed a long list of other companies in being hacked this holiday season. At least one Twitter user, @KingEbola, was claiming credit for the outage while it was ongoing:

Ryan Gorman (@GormoJourno) The Twitter account claiming to be responsible for the outage, “KingEbola," has tweeted another demand. pic.twitter.com/GyjbAGPjK6

But the screenshot posted by @KingEbola seems to be dated to May 2012, and now that the service is back up, their account has been wiped of all but a single tweet.

Hopefully the real New Year’s Eve will be more pain-free for Twitter, especially considering it’s usually one of the biggest nights of the year worldwide. In Japan in particular, where users tend to race to send “Happy New Year” tweets on the dot of midnight, the tweets per second reach dizzying highs – so high, in fact, that in 2012, the service was brought down by the volume.