MENLO PARK (AP) — Facebook’s making some of its users feel a lot older than they really are.

The social network sent automated messages Thursday inviting some users to celebrate “46 years of friendship on Facebook” with one or more of their online friends.

That’s odd, since Facebook only started in 2004. And some people who got the message are in their 20s and 30s.

Facebook blamed a software bug. It didn’t offer details, but computer experts speculate the problem stems from a quirk in Unix, an operating system used in big data centers where companies like Facebook store information. The glitch starts the internal calendar on some computers at midnight, Greenwich Mean Time, Jan. 1, 1970.

Facebook said its engineers are fixing the problem “so everyone can ring in 2016 feeling young again.”