Every computer system needs a "time zero"—a reference point from which to count. This is known as an epoch. While many exist, the most famous is the Unix Epoch: January 1, 1970. This article explores why this date was chosen and how it differs from other critical epochs in computing.

The Unix Epoch (Jan 1, 1970): Why It Became the Standard

The choice of 1970 was largely a matter of convenience for the creators of the Unix operating system in the early 1970s. They needed a starting point, and January 1, 1970, was a clean, recent, and arbitrary round number to begin counting from. Because of Unix's immense influence, this epoch became the de facto standard for a generation of software.

Beyond Seconds: Milliseconds, Microseconds, and JavaScript Time

A critical point of confusion for developers is the difference in precision. A traditional Unix timestamp is a 10-digit number representing seconds since the epoch. However, modern languages like JavaScript (`Date.now()`) and Java use a 13-digit number representing milliseconds since the same epoch. This factor-of-1000 difference is a common source of bugs.

Other Epochs: GPS and Truncated Julian Date (TJD)

Not all systems use 1970. Other important epochs include:

• GPS Time: The GPS system started its clock at midnight on January 6, 1980.
• Truncated Julian Date (TJD): Used by NASA and other space agencies, this is a simplified day count that started on May 24, 1968.

Debug Your Epoch Time Errors

Understanding which epoch and which unit of time (seconds, milliseconds, etc.) your system is using is the first step to debugging any timestamp-related issue.