1700000000. Ten digits that mean "November 14, 2023" to every computer on Earth. Unix time — seconds counted from midnight UTC on January 1, 1970 — is the closest thing software has to a universal language for "when." Here's how it works and where it breaks.
Why 1970, and why seconds
The epoch date is a historical artifact: early Unix engineers picked a round, recent starting point, and it stuck. Seconds were the natural unit on 1970s hardware. The format's genius is what it leaves out — no time zones, no daylight saving, no calendar quirks. An epoch timestamp is a single integer, trivial to store, compare, and sort. Time zone math happens only at display time, where it belongs.
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Modern systems often use milliseconds (13 digits) or microseconds instead, because second-precision is too coarse for event ordering. If your "timestamp" has 13 digits, divide by 1,000 — you're looking at millis.
The 2038 problem
On January 19, 2038, at 03:14:07 UTC, the epoch second counter exceeds what a signed 32-bit integer can hold (2,147,483,647) and wraps to a negative number — landing in December 1901. Any system still storing time in 32 bits will think it's over a century ago.
Modern 64-bit platforms are fine (their clocks run for billions of years), but 32-bit timestamps survive in embedded systems, old file formats, network protocols, and databases with INT time columns. 2038 is a Y2K for infrastructure — less visible, longer-tailed, and absolutely worth auditing for now rather than later.
Leap seconds: the asterisk
Unix time pretends every day has exactly 86,400 seconds. Reality disagrees — Earth's rotation wobbles, so UTC inserts leap seconds. Unix time handles them by repeating a second, which means epoch timestamps can't represent leap seconds distinctly and naive "seconds between two dates" arithmetic drifts by ~27 seconds over the decades. For scheduling and logging this never matters; for astronomy and high-frequency trading, it very much does.
Reading epoch values yourself
When an API hands you 1735689600 or a log file is full of 13-digit numbers, paste them into the epoch timestamp batch converter — it converts whole lists at once, in both directions, without sending your data anywhere. Learning to eyeball epoch values (1.7 billion ≈ mid-2020s) is also a genuinely useful debugging skill.
