The conventional civil timescale used in most parts of the world is based on Coordinated Universal Time (UTC sic). UTC is based on International Atomic Time (TAI sic), which is derived from hundreds of cesium clocks in the national standards laboratories of many countries. Deviations of UTC from TAI are implemented in the form of leap seconds, which occur at intervals from several months to several years. This page considers what happens in the NTP timescale upon the epoch of a leap second. While TAI is useful in the scientific community, most of us reckon time according to the Sun and season. Starting from TAI, the UT0 timescale is determined using corrections for Earth orbit and inclination (the Equation of Time as used by sundials). The International Earth Rotation Service (IERS) at the Paris Observatory uses astronomical observations provided by USNO and other observatories to determine the UT1 (navigator's) timescale corrected for irregular variations in Earth rotation.