Year 2038 Problem

On January 19th 2038 at 03:14:08 UTC many computer programs will inadvertently stop working. Some will crash, others will report the time as December 13th 1901 at 20:45:52 UTC. A flashback to Y2K, we'll have Y2k38.

Why? Many programs measure time in Unix time, which is the number of seconds elapsed since the Unix epoch (January 1st 1970 00:00:00 UTC). The current epoch time of this post is 1,646,058,600. On January 19th 2038, Unix time will reach an important number to computer scientists, 2,147,483,647.

2,147,483,647 can also be written as 2³¹ − 1. Unix timestamps usually are stored in 32-bit signed integers. A 32-bit signed integer uses the first bit to determine the sign, and the next 31 to encode the number, so it can represent integers from −(2³¹) to 2³¹ − 1.

When a 32-bit signed integer overflows, or goes beyond its maximum value, it flips the signed bit, or −(2³¹). Unix timestamps stored in 32-bit integers that overflow will read −(2³¹), or 2³¹ seconds before epoch – December 13th 1901.

Many programs have already safeguarded against this by storing Unix timestamps in a 64-bit integer (that kicks the problem 292 billion years into the future), but some legacy programs will inevitably fail in unexpected ways. We'll have a lot more legacy software in 2038 than we did in 2000 (Y2K).