Greenwich Mean Time (GMT) is an old name for (Coordinated) Universal Time Clock (UTC) it's called Greenwich Mean Time because it's the time at Greenwich in England Time Zone 0 all other time zones are derived from this time zone in Greenwich. In Sydney, Australia for example the time zone is +10 so it's ten hours ahead of this time in Greenwich the time in California, America for example is time zone -7 it's seven hours behind this time in Greenwich. Greenwich is actually 51 degrees long and 0 degrees latitude and some have a problem with that because this time should come from 0 degrees longitude and 0 degrees latitude a place known as "Null Island" and this version of the Universal Time Clock (UTC) is sometimes referred to as UT0 or UT1 but to the military, "Zulu Time".