Why Twitter was down for 5 hours. It’s all about calendars.


Twitter was down for many users this Monday, between midnight and 5 AM, CET. Android app users were logged out and the API was not working during a few hours. A bug in a line of code caused the service to think that it was 29 December, 2015.

Gregorian VS ISO calendars

The ISO week numbering system uses the YYYY format for the year instead of the Gregorian calendar’s yyyy. It then looks at which week of the year it is, and then uses a date digit with 1 starting on Monday. So, for example, Tuesday of the 50th week of 2014 would have been 2014-W50-2 in ISO week format. The problem comes in when January 1 of the new year ends up falling on a date that doesn’t get along well with the ISO week format. The first day of 2015 will start on a Thursday, whereas the ISO standard expects the first week of the year to start on “the Monday that contains the first Thursday in January.” In 2014-2015, that would be January 1st, 2015. That’s why Twitter believed that we were in 2015 on Monday, because “the first Thursday in January” is in fact this week.

Afterwards, some parts of the system believed we were almost a year later, while other were perfectly aware that we were still in 2014. Users were not able to log in (and were logged out) because tokens have a limited lifetime and therefore are not valid a year later.

Dates are hard to handle

Dates are really hard to handle when programming, and it’s very easy to make a mistake without breaking a thing until the day it will eventually explode everything because calendars or timezones are different. The same kind of mistake happened two years ago with the “Do Not Disturb” feature on iPhones, and it isn’t the last time it will happen for sure.

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