The International Atomic Time scale (TAI) and Universal Coordinated Time (UTC) are two different answers to the same question — "what time is it, exactly?" — and the fact that they diverge by a growing number of seconds (currently 37 seconds) explains a discontinuity at the heart of timekeeping that affects everything from GPS satellites to financial transaction timestamps to the software that had to handle a 23:59:60 last minute of the year
Most people encounter time zones and clock conversions as the visible complexity of timekeeping. Beneath that layer is a more fundamental complexity: the definition of a second itself, and the impossible requirement to simultaneously honour both the precision of atomic physics and the astronomical reality that the Earth's rotation is not constant.
Two definitions of a second, and why they conflict
The SI second (since 1967): defined as exactly 9,192,631,770 oscillations of the caesium-133 atom's hyperfine transition. Constant, reproducible anywhere in the universe, drifts by less than 1 second per 300 million years. This is what atomic clocks measure.
The astronomical second (traditional): 1/86,400 of a mean solar day — the time for the Earth to complete one rotation relative to the Sun. The problem: the Earth's rotation is not constant. Tidal friction from the Moon is gradually slowing Earth's rotation by approximately 1.4 milliseconds per century. The mean solar day is currently approximately 86,400.002 SI seconds long — a 2-millisecond difference that accumulates.
The conflict: if clocks run on atomic SI seconds forever, solar noon (the moment when the Sun is highest in the sky) would drift away from 12:00:00 — eventually arriving at 11:50, 11:00, and eventually at night. The astronomical community and maritime navigation historically required clocks to track solar time. Atomic physics communities required exact SI-second precision.
Leap seconds: the compromise
UTC (Coordinated Universal Time) was designed as a compromise between atomic precision and astronomical coupling. It runs on atomic SI seconds (perfectly precise), but periodically inserts or (theoretically) deletes a "leap second" to keep UTC within 0.9 seconds of UT1 (the astronomical time scale tracking Earth's actual rotation).
How leap seconds are inserted: announced by the IERS (International Earth Rotation and Reference Systems Service) approximately 6 months in advance. On the scheduled date (historically always June 30 or December 31), the UTC clock shows:
23:59:58
23:59:59
23:59:60 ← the leap second (an additional second that doesn't normally exist)
00:00:00
The divergence from TAI: International Atomic Time (TAI) just runs SI seconds continuously with no leap seconds. UTC = TAI − 37 seconds (as of 2024). Every time a leap second is added to UTC, this offset increases by 1 second.
When the next adjustment occurs: the IERS announced in 2022 that leap seconds will be discontinued by 2035. After that, UTC will be allowed to drift from UT1 by up to a minute (instead of the current 0.9-second tolerance), with a "leap minute" eventually needed — a more drastic future problem deferred.
Software systems that failed to handle leap seconds
The June 30, 2012 leap second caused widespread failures across the internet:
Reddit: experienced a Java application freeze caused by a bug in how Linux's ntpd (time synchronisation daemon) handled the extra second. Some kernel versions set a CPU flag (hrtimer_interrupt) that caused a loop that pegged CPUs at 100%, making servers unresponsive.
Mozilla: similar Linux kernel CPU pegging issue. Multiple Mozilla services degraded or went offline.
LinkedIn: affected by the same Linux kernel issue.
Amadeus (airline reservation system): experienced booking delays due to time-related processing issues.
The technical root cause: the Linux kernel (and ntpd) had a bug in handling the 23:59:60 second that caused the hrtimer (high-resolution timer) subsystem to incorrectly interpret the extra second, triggering a CPU spin loop.
The broader problem for software: POSIX time (the time standard used by Unix/Linux) specifies that a day always has exactly 86,400 seconds — which means POSIX doesn't represent leap seconds. When a leap second occurs, POSIX-compliant systems either repeat a second (23:59:59 appears twice) or "smear" the leap second across a longer period (Google and Amazon both implement "leap second smearing" where each second in a 24-hour window is slightly longer than usual, distributing the extra second invisibly).
GPS time and the TAI connection
GPS satellites maintain their own time scale called GPS Time — which, unlike UTC, has no leap seconds. GPS Time was synchronised with UTC at midnight January 6, 1980 (when the offset was zero). Since then, UTC has accumulated 18 leap seconds that GPS Time has not. Therefore:
GPS Time = UTC + 18 seconds (as of 2024; this was 0 in 1980)
GPS receivers and navigation systems must know the current UTC-GPS offset (broadcast in the GPS signal) to display correct UTC time. A GPS receiver that doesn't correctly handle this offset could show time that's 18 seconds off — significant for applications that use GPS as a time source.
The navigation calculation: GPS positioning uses time differences between signals from multiple satellites to calculate position. These differences need to be accurate to nanoseconds. The leap second offset between GPS Time and UTC doesn't affect positioning calculations (which use GPS Time consistently) but must be applied correctly when converting GPS Time to the UTC time displayed to users.
Time in finance: why microsecond timestamps matter legally
Financial regulators have increasingly strict timestamp requirements for trade reporting:
MiFID II (EU, 2018): requires timestamps for trade reports accurate to 1 microsecond (0.000001 seconds) for algorithmic trading. Human-executed trades require 1-millisecond accuracy.
SEC Rule 613 (US, 2020): Consolidated Audit Trail requires timestamps accurate to 1 millisecond (0.001 seconds) for trade data.
The leap second problem in finance: a leap second creates a moment when timestamps may be duplicated or skipped, depending on how the exchange's timekeeping system handles it. Two distinct events that occur at 23:59:59 in different systems — one before the leap second is applied and one after — may be timestamped identically in UTC, creating audit trail ambiguities. This is one reason exchanges develop specific leap second policies and notify participants in advance.
How to use the Time Converter on sadiqbd.com
- For computing contexts: convert between human-readable time units and the units used in programming (milliseconds, microseconds, nanoseconds) — particularly for timeouts, rate limits, and cache TTLs where the difference between "1000 milliseconds" and "1 second" is semantically identical but the unit mismatch has caused real bugs
- For academic and productivity calculations: convert between calendar years, work years (approximately 2,000 hours for a standard work year), and hours to contextualise claims like "10,000 hours of practice" or project time estimates
- For leap second awareness: when calculating elapsed time across the boundaries of known historical leap second dates (available at the IERS website), add the appropriate correction if your calculation must be precise to better than 1 second
Frequently Asked Questions
If leap seconds are being abolished after 2035, what will happen to the UTC-UT1 difference over the next century? It will grow, until a "leap minute" becomes necessary — estimated sometime in the 22nd century. After 2035, the new agreement allows UTC to drift from UT1 by up to 1 minute rather than 1 second. The Earth's rotation slowdown is gradual enough that accumulating a full minute of drift takes roughly 50-100 years after the 0.9-second tolerance is abandoned. The 2022 CGPM (General Conference on Weights and Measures) decision to abolish leap seconds specifically defers — not eliminates — the problem. The engineering challenge of inserting a leap second (causing widespread software failures) will eventually be replaced with the even larger engineering challenge of inserting a leap minute — though the longer time horizon should allow for better software preparation.
Is the Time Converter free? Yes — completely free, no sign-up required.
Try the Time Converter free at sadiqbd.com — convert between seconds, minutes, hours, days, weeks, months, and years instantly.
