Epoch

In timekeeping, an epoch is a reference point in time used as the origin for a particular time measurement system. Different systems use different epochs as their starting points for counting time.

Some notable epochs include:

1. Unix Epoch: January 1, 1970, 00:00:00 UTC - Used for Unix timestamps, which count the number of seconds since this moment.

2. GPS Epoch: January 6, 1980, 00:00:00 UTC - The starting point for GPS time, which counts weeks and seconds within each week.

3. J2000 Epoch: January 1, 2000, 12:00:00 TT (Terrestrial Time) - A standard reference epoch used in astronomy and space science.

4. Microsoft Windows Epoch: January 1, 1601, 00:00:00 UTC - Used in NTFS file systems and some Windows APIs.

5. NTP Epoch: January 1, 1900, 00:00:00 UTC - Used in the Network Time Protocol.

Epochs are chosen for various practical or historical reasons. For example, the Unix Epoch was chosen to be compatible with the limitations of 32-bit integers when Unix was developed, while astronomical epochs are often chosen to coincide with significant astronomical events or periods.

When working with different time systems, it's important to be aware of their respective epochs to correctly convert between them.