Epoch

Overview

In computing, an epoch is the reference point from which time is measured. The most widely known is the Unix epoch: January 1, 1970, at 00:00:00 UTC. All Unix timestamps are expressed as the number of seconds (or milliseconds) elapsed since this moment. Different systems use different epochs, which can cause confusion when exchanging time data between platforms.

Notable Epochs

Windows uses January 1, 1601 as its epoch (the start of a 400-year Gregorian calendar cycle). Apple's Cocoa framework uses January 1, 2001. GPS time starts from January 6, 1980. The NTP epoch is January 1, 1900. When converting between systems, developers must account for these different starting points to avoid incorrect time calculations.

Why 1970?

The Unix epoch of January 1, 1970 was chosen pragmatically. Early Unix systems used a 32-bit counter for seconds, and 1970 was recent enough to be useful while providing sufficient range into the future (until 2038 for signed 32-bit). The date itself has no astronomical or historical significance; it was simply a convenient round number that balanced past and future range for the hardware limitations of the era.