Epoch = 00:00:00 UTC on 1 January 1970. Timestamps count seconds since then, ignoring leap seconds.
Digits tell you the unit: 10 digits ≈ seconds, 13 ≈ milliseconds (JavaScript Date.now()), 16 ≈ microseconds, 19 ≈ nanoseconds.
Year 2038: signed 32-bit seconds overflow at 2,147,483,647 (03:14:07 UTC, 19 Jan 2038).
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