Why Is Unix Epoch 1970

Operating Systems

Have you ever wondered why the Unix epoch starts at 1970? It’s a question that has intrigued many computer enthusiasts, including myself. In this article, I will delve into the history and technical reasons behind this seemingly arbitrary choice.

The Birth of Unix

In order to understand why the Unix epoch is set to 1970, we need to go back to the birth of Unix itself. Unix, developed by a team at Bell Labs in the late 1960s, was one of the first operating systems to be written in a high-level programming language, namely C. This made Unix highly portable and allowed it to be easily adapted to different hardware architectures.

When Unix was being developed, the team needed a way to represent and manipulate time within the operating system. They decided to do this by measuring time as the number of seconds that have elapsed since a specific point in time. But what point in time should they choose?

The Start of Time

After careful consideration, the Unix team settled on January 1, 1970, as the starting point for measuring time. This date, known as the Unix epoch, was chosen for several reasons.

Firstly, 1970 was a significant year in computing history. It was the year that saw the release of the first commercially available microprocessor, the Intel 4004. This marked a major milestone in the development of computer hardware, and the Unix team wanted to align their operating system with this new era of computing.

Secondly, January 1, 1970, was a convenient starting point from a technical perspective. It fell on a Thursday, which made it easy to calculate the day of the week for any given date. This was important for scheduling and other time-related operations within Unix.

The 32-bit Limitation

Another factor that influenced the choice of the Unix epoch was the limited amount of storage available at the time. In the late 1960s and early 1970s, computer systems had a limited amount of memory and disk space, and every byte counted.

The Unix team decided to use a 32-bit signed integer to represent the number of seconds since the Unix epoch. This meant that the maximum representable date in Unix time would be January 19, 2038, due to the “Year 2038 problem.” Choosing January 1, 1970, as the epoch allowed for a reasonable range of representable dates without wasting precious storage space.


In conclusion, the choice of 1970 as the Unix epoch was influenced by the historical significance of the year, technical considerations related to computing hardware and storage limitations, and the need for a consistent and easily calculable starting point for time measurement within the Unix operating system.

So the next time you encounter the Unix epoch, you’ll know the story behind why it all began in 1970.