BIPM decides to stop adding leap seconds to UTC
International timekeepers have decided to stop adding leap seconds to Coordinated Universal Time (UTC) from 2027 onward, allowing it to gradually drift from Earth’s rotational (solar) time. Commenters largely welcome the change as a practical move that simplifies software, networking, and timekeeping systems, which have long struggled with rare, irregular 61-second minutes and the prospect of negative leap seconds. The remaining gap with true solar time can be handled in the far future by shifting time zones or, in theory, adding a “leap hour,” but many argue that by then civil time will be even more decoupled from the Sun.
Overview of the Decision
- BIPM will stop inserting leap seconds into UTC starting May 2027; the 27 existing leap seconds remain.
- UTC will effectively be TAI plus a fixed offset (currently 37 seconds), gradually drifting from Earth rotation time (UT1).
- The resolution allows UT1–UTC to grow up to one hour; what to do then (e.g., leap hour vs timezone shifts) is explicitly left to future generations.
Arguments in Favor
- Leap seconds are a frequent source of bugs: non‑monotonic timestamps, duplicated seconds, unsupported “23:59:60”, and inconsistent handling (step vs smear vs ignore).
- Dropping leap seconds simplifies Unix/POSIX time, making the mapping between UTC and elapsed atomic time stable and predictable over centuries.
- Many systems and languages already assume 86,400 seconds per day and ignore leap seconds; practice never matched the spec.
- Negative leap seconds have never occurred and are seen as especially risky because almost no software paths are proven for them.
- For most people, precise solar alignment is irrelevant: time zones are coarse, DST shifts an hour at a time, and solar noon already varies ±30 minutes seasonally and geographically.
- Astronomical/solar-accurate work can use UT1 and explicit UT1–UTC corrections without having to “undo” leap seconds first.
Concerns and Critiques
- Some view this as a “defeat” or lazy complexity reduction, abandoning the original UTC promise of tightly bounding solar deviation.
- Critics argue we already have a monotonic scale (TAI) and should have left UTC as the solar-aligned standard instead of redefining it.
- Existing UTC code paths for leap seconds become dead or historical; future systems must distinguish “old UTC with leap seconds” from “new UTC without.”
- Precision users (e.g., celestial navigation, geodesy) that previously relied on UTC’s bounded offset from UT1 must now track UT1–UTC explicitly.
Long-Term and Philosophical Views
- Several commenters note that any leap hour, if ever needed, could be absorbed by timezone changes, like DST shifts, rather than touching UTC.
- Many expect that thousands of years from now, civil time will be fully decoupled from Earth’s rotation, especially for a spacefaring civilization.
- There is both enthusiasm (“good riddance to leap seconds”) and worry that future entities will see this as offloading problems onto them.