Time scales and civil time
UTC
Also: Coordinated Universal Time
UTC is the time scale on which civil time worldwide is based. It combines atomic time (TAI, very stable) with the rotation of the Earth: leap seconds are inserted from time to time so that UTC never differs by more than 0.9 seconds from astronomical time (UT1).
Time zones are expressed as an offset relative to UTC. Belgium uses UTC+1 in winter and UTC+2 in summer.