Facebook '46 years' notifications: Why social network told users they had been friends for decades, and why it will happen again

The strange notifications were the result of a bug in Unix, which lays the foundation of modern operating systems

Facebook has been telling people that they have been friends for 46 years — and apparently for no reason.

Users on the site have been seeing messages telling people that they’ve been friends with people on the service for 46 years. The mix-up appears to applying indiscriminately.

But the strange behaviour is the result of a computer glitch that happens occasionally for little reason, and could well happen again.

Engineers online have speculated that the problem all arises from something known as the “Unix Epoch”. That is the time that all computers that use the widely-shared system count from, and it appears to have had a mix-up while deciding how long ago people became friends.

Computers that use the system count up from a fixed date — 1 January, 1970, which is known as the Unix Epoch. Rather than considering time as a date, computers think of it as a certain number of days past that date.

But the problem appears to be arising from the fact that some people became friends before the feature was introduced. As a result, Facebook doesn’t have information about when it happened, and its database records the value as simple “0”.

The issue comes when the system looks in that database and sees the number zero. Because of the way that time works, it thinks of that as indicating no days, which would have begun at the Unix Epoch — which is 46 years ago.

Computer programmers have a way of dealing with the problem, by telling the computer that if it sees a zero it should exclude them from its search. But that check appears to have been left out, leading to the problems.

The issue could well appear again, because it is the result of a simple omission. Any computer system that works with time and has the potential not to know how long ago something happened could throw up the same strange results.

Facebook didn’t say exactly why the problem happened. But it did say that it was working to fix the feature, “so everyone can ring in 2016 feeling young again”.