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g M t
The abbreviation "GMT" stands for Greenwich Mean Time and
refers to mean solar time at the Greenwich meridian crossing
the royal observatory near London and which serves as the basis
for calculating longitudes. "Greenwich Mean Time" served as a
temporal reference until 1972, the year when it was replaced by
Universal Time Coordinated (UTC). Despite this new reference,
the term GMT is still widely used.
Historically speaking, the surface of the globe had been divid-
ed into 24 equal-sized time zones, in which the hour was iden-
tical throughout each. The dateline had initially been fixed at
the 180° meridian. This subdivision was subsequently modified
to take account of the needs of certain countries wishing for
example to have only one legal time, despite the fact that their
territory extends over several time zones (such as China) or
that their legal time should be different from that theoretically
allocated (e.g. Spain). Moreover, some countries (such as Iran)
have adopted a legal time corresponding to 30 and 45-minute
fractions of a time zone.
By now there are actually 41 time zones, including some that
exist only temporarily (Daylight Saving Time or "summer" and/
or winter time, southern and/or northern hemisphere); among
them 36 are valid year-round.
The choice of the dateline corresponds approximately to the
180° meridian, located in the Pacific Ocean and distinguished
by the fact that it touches virtually no land. The line has none-
theless been slightly diverted to avoid two islands.
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pdr 4007 (gMt - p
)
alibre
ower reserve
The Pierre DeRoche GMT watch with its mechanical self-
winding movement enables one to handle two time-zone indica-
tions, in addition to the hour, minute, seconds and date display.
The originality of this mechanism is further enhanced by the
addition of a power-reserve indication.
It is worth noting that the date display is coupled with the GMT
mechanism; in other words, the date is synchronised with the
so-called "local" time shown by the watch. The date indicated
by the calendar therefore jumps "forwards" or "backwards"
when local time goes past midnight during operation of the
GMT push-button correctors.
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