Coordinated Universal Time (UTC) is a high-precision atomic time standard. UTC has uniform seconds defined by International Atomic Time (TAI), with leap seconds announced at irregular intervals to compensate for the earth is slowing rotation, and other discrepancies. The leap seconds allow UTC to closely track Universal Time (UT), which is a time standard based on the earth is angular rotation, rather than a uniform passage of seconds.
Time zones around the world are expressed as positive or negative offsets from UTC. In this role as the zero-point reference, UTC is also referred to as Zulu time (Z). UTC is often referred to as Greenwich Mean Time when describing time zones, although strictly speaking UTC (atomic time scale) is only approximately the same as GMT.