Abbreviation for Coordinated Universal Time. Basis for expressing worldwide time data. The UTC reference time is not a local time zone, but a general reference time based on atomic time standards, from which the times of all local time zones are derived. UTC is defined in such a way that it corresponds to Greenwich Mean Time (GMT). The UTC reference time differs slightly from Universal Time (UT1), which is linked to the rotation of the earth and defined as the 86,400th part of the mean solar day. By using leap seconds, this difference is kept down to less than 0.9 seconds a year.