Why Unix Time Counts From 1970, and the Millisecond Trap
Unix time counts seconds elapsed since midnight UTC on January 1, 1970 (the "Unix epoch") — an arbitrary but now deeply entrenched reference point chosen by early Unix developers, with no deeper significance than needing some fixed starting point for a simple integer-based time representation.
The single most common Unix timestamp bug: confusing seconds since epoch with milliseconds since epoch. Many systems and APIs (notably JavaScript's Date.now()) return milliseconds, not seconds, and passing a millisecond value to a function expecting seconds (or vice versa) produces a timestamp off by a factor of 1000 — which typically manifests as a date wildly in the past (near 1970) or absurdly far in the future, an unmistakable but sometimes puzzling-to-diagnose symptom if you don't immediately suspect a units mismatch.
Unix timestamps are inherently timezone-agnostic — the integer itself represents a specific instant in time regardless of timezone; only the display of that instant as a human-readable date depends on which timezone you choose to render it in, which is exactly why the same timestamp can correctly show different calendar dates and times depending on the viewer's location.