.TH GMT "" "" "Definition"
.PC 
.PP
.B GMT
is an abbreviation of Greenwich Mean Time,
the time recorded at the Greenwich Observatory in
England, where by international convention the Earth's zero
meridian is fixed.
.PP
By definition, \*(PN fixes system
time in GMT.
It calculates local time
as an offset of GMT; for example, the time zone for
Chicago is six hours (360 minutes) behind Greenwich, so the local
time for Chicago is calculated by subtracting 360 minutes from GMT.
.SH "See Also"
.Xr "gmtime()," gmtime
.Xr "localtime," localtime
.Xr "Programming COHERENT," programmin
.Xr "time," time
.Xr "time.h," time.h
.Xr "TIMEZONE" timezone
.SH Notes
.II UTC
The \*(AS replaces GMT with UTC
(\fIuniversel temps coordonne\fR
or universal coordinated time) for C programming.
The change is mainly one of terminology rather than substance,
as some signatories to international conventions
.\" i.e., those who eat snails and frogs
object to naming the standard for global time after a suburb of London.
.PP
Under international convention, there are two UTC standards:
UTC1 is based on solar time and is identical to current GMT; and
UTC2, which uses atomic clocks that are corrected by comparison with pulsars.
These standards drift apart as the earth's rotation slows; thus,
``leap seconds'' are inserted periodically into UTC1 to bridge
the difference.
