UTC vs. TAI

International Atomic Time (TAI) is what it sounds like: a timescale defined purely by counting oscillations of atomic clocks, with no reference to the Earth's rotation at all. Coordinated Universal Time (UTC) — the timescale behind every civil clock and the basis for Unix time — is built from the same atomic seconds, but periodically adjusted with leap seconds to stay loosely aligned with the actual solar day.

Why the gap is exactly 37 seconds

TAI and UTC were defined to match on January 1, 1972, but UTC has had 27 leap seconds inserted since then (see the complete list), plus an original 10-second offset baked in at the start — 10 + 27 = 37 seconds, the current, stable gap between the two as of this page's last update. That gap only grows if another leap second is added; it never shrinks, since (so far) every leap second has been positive.

Why most systems use neither directly

Unix time deliberately ignores leap seconds entirely — every day is defined as exactly 86,400 seconds, even the handful of days that technically had 86,401 real atomic seconds. That makes Unix time closer to UTC in spirit (tracking civil time, roughly aligned with the sun) while being mathematically simpler than either UTC or TAI to do date arithmetic with. Systems that need real, unambiguous elapsed-time counting — GPS satellites, some scientific timing systems — use TAI or GPS time (a close cousin, itself offset from TAI by a fixed 19 seconds) specifically to avoid the leap-second discontinuities UTC has by design.

The takeaway for working developers

Almost nothing you build needs TAI directly — Unix time (via UTC) is the right choice for the overwhelming majority of software. The exception is anything doing precise duration math across a leap-second boundary, where Unix time's leap-second-blindness can introduce a one-second discrepancy that TAI-based timing avoids by design.