# Leap Second Table — IERS DUT1, TAI-UTC Offset & Cross-System Time IERS leap-second history (27 insertions since 1972), current TAI-UTC offset (+37 s), DUT1 (UT1-UTC) notes, and POSIX/NTP/GPS/TAI/UT1 cross-system conversion for a given UTC instant > Canonical page: https://elysiatools.com/en/tools/leap-second-table-iers-dut1-tai-utc-offset-diff - **Category:** Date & Time - **Keywords:** leap second, iers, dut1, tai-utc, ut1, atomic time, gps time, posix, ntp, bulletin c, time scale ## Overview For satellite ground-station operators, astronomers, HFT clock-sync engineers, and GNSS researchers. Shows the 27 IERS leap-second insertion events since 1972-01-01 and the cumulative TAI-UTC offset (currently +37 s, unchanged since 2017-01-01). Conversion mode: enter a UTC instant (and an optional DUT1 estimate) to get TAI = UTC + offset, GPS = UTC + (offset−19), UT1 = UTC + DUT1, the Unix/POSIX timestamp, and the leap-second status for that instant. Data from IANA leap-seconds.list / NIST / IERS. Pure local, deterministic, no network — DUT1 is live data (Bulletin A, weekly); here it is an estimated input, consult the official bulletin for the precise value. ## Inputs - **Mode** (select) - **UTC instant (ISO 8601)** (text): e.g. 2026-08-10T12:00:00Z (used in conversion mode) - **DUT1 estimate (s)** (number): UT1−UTC in seconds, e.g. 0.1. Leave 0 if unknown. ## When to use - Review the 27 IERS leap-second insertion events and cumulative TAI-UTC offsets since 1972. - Convert a UTC timestamp for satellite, astronomy, GNSS, or clock-synchronization work. - Compare civil, atomic, GPS, and Earth-rotation time scales for a historical or current instant. ## How it works - Select "Leap-second history table" to view the IERS insertion events and associated TAI-UTC offsets. - Select "Cross-system time conversion" and enter a UTC instant in ISO 8601 format. - Optionally enter a DUT1 estimate in seconds; leave it at 0 when the value is unknown. - The tool calculates TAI, GPS, UT1, Unix/POSIX time, and the leap-second status using the applicable historical offset. ## Use cases - Satellite ground-station teams can inspect leap-second history and compare UTC with TAI, GPS, and UT1. - Astronomers and GNSS researchers can evaluate historical time-scale offsets for a specified UTC instant. - Clock-synchronization engineers can check Unix/POSIX and atomic-time relationships without relying on a network connection. ## Frequently asked questions ### What is the current TAI-UTC offset? The current cumulative TAI-UTC offset is +37 seconds and has been unchanged since 2017-01-01. ### What is DUT1? DUT1 is the difference UT1−UTC, entered here as an estimate in seconds. ### What UTC format should I enter? Enter an ISO 8601 instant, such as 2026-08-10T12:00:00Z. ### Which time scales does conversion include? Conversion includes TAI, GPS, UT1, and Unix/POSIX time, along with leap-second status. ### Does the tool retrieve live DUT1 data? 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