What is the Unix epoch and why does it start in 1970?

The Unix epoch is the reference point for Unix time: midnight on January 1, 1970, UTC. A Unix timestamp expresses any moment as the number of seconds elapsed since that epoch. The year 1970 was chosen by the early Unix developers at Bell Labs as a round number close to when the system was being developed; 1970 predates the earliest Unix systems by a few years, giving negative timestamps for moments before the epoch. The choice of seconds as the unit was a practical decision for the memory constraints of 1970s hardware. Today, Unix time is ubiquitous across operating systems, databases, programming languages, and network protocols.

What is the Year 2038 problem?

The Year 2038 problem (Y2K38) affects systems that store Unix timestamps as a signed 32-bit integer. A signed 32-bit integer can represent a maximum value of 2,147,483,647, which corresponds to January 19, 2038, at 03:14:07 UTC. One second later the value overflows to a large negative number, representing a date in 1901. Systems using 64-bit integers to store timestamps are not affected; a 64-bit signed integer can represent dates approximately 292 billion years from the epoch. Modern 64-bit operating systems and most programming languages have already addressed this, but embedded systems, legacy databases, and older file formats may still be at risk.

Does Unix time count leap seconds?

No. Unix time deliberately ignores leap seconds, treating every day as exactly 86,400 seconds. Leap seconds are occasionally inserted by the International Earth Rotation and Reference Systems Service (IERS) to keep UTC aligned with UT1 (astronomical time based on Earth's rotation), which slows unpredictably. Between 1972 and 2024, 27 leap seconds were added. Because Unix time skips them, there is a 27-second discrepancy between Unix time and TAI (International Atomic Time). Network Time Protocol (NTP) servers handle leap seconds by slowing or repeating the last second of the day, a technique called 'leap smearing' used by Google and Amazon.

What is the difference between Unix time in seconds and milliseconds?

Original Unix time is measured in seconds, producing a 10-digit number for dates in the current era (e.g., 1700000000 for November 2023). JavaScript's Date.now() and many web APIs return milliseconds since the epoch, producing a 13-digit number. Some systems use microseconds (16 digits) or nanoseconds (19 digits). When parsing an unknown timestamp, count the digits: 10 digits = seconds, 13 = milliseconds. Division by 1000 converts milliseconds to seconds. Mixing up seconds and milliseconds is a common source of bugs that causes dates to appear in 1970 or in the far future.

How do I find the Unix timestamp for 'right now'?

In most environments: Python uses `import time; time.time()`, JavaScript uses `Date.now()` (milliseconds) or `Math.floor(Date.now()/1000)` (seconds), bash uses `date +%s`, and SQL uses `EXTRACT(EPOCH FROM NOW())` in PostgreSQL or `UNIX_TIMESTAMP()` in MySQL. Command-line tools like `date -u +%s` on Linux give the current UTC epoch in seconds. Network Time Protocol (NTP) synchronizes system clocks to within a few milliseconds of UTC, so these values are reliable for timestamps as long as the system clock is NTP-synced.

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注目のUnixタイムスタンプ

イベント	タイムスタンプ	日付（UTC）
Unixエポック	0	1970-01-01 00:00:00
10億	1,000,000,000	2001-09-09 01:46:40
Y2K38問題	2,147,483,647	2038-01-19 03:14:07
20億	2,000,000,000	2033-05-18 03:33:20

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