Epoch time is a method of representing time and dates by counting the number of seconds elapsed since a start date. This start date is referred to as the beginning of an epoch. Computer systems use epoch time to track the current time and date and to set created and modified timestamps.
Since most computer servers use Unix or a Unix-like operating system, the term "epoch time" is most often used to refer to the Unix epoch that began at 12:00:00 AM UTC on January 1, 1970. However, other computer systems track time using other starting dates. Classic MacOS started its epoch at midnight UTC on January 1, 1904; modern Windows systems track epoch time from a start date of January 1, 1601. Windows computers also increment epoch time every 100 nanoseconds instead of every second.
Errors in a file or file system may occasionally reset a file's created or modified date to 0. When this happens, your computer's operating system will display the affected file as created at midnight UTC on January 1, 1970. Coordinated Universal Time (UTC) is based on Greenwich Mean Time (GMT), so if your local time zone is west of GMT, the date adjusts a few hours back to December 31, 1969.
NOTE: Unix systems that still use a signed 32-bit integer to track epoch time will reach the maximum possible value on January 19, 2038. Updating affected software to use a 64-bit value instead will resolve this problem since a 64-bit value can track more than 500 billion years worth of seconds.