What is the deal with GMT?

Subject: What is the deal with GMT? Wed Apr 14, 2010 6:21 pm

I found this summary at http://www.larrybiggs.net/mirror/gmt/gmt24.htm and wanted to pass it along. The following is a very good summary from Larry Biggs that goes through the explanation of what a GMT watch does and how to use it. Take a read....

INTRODUCTION

Wrist watches with a dual-time reading are often linked with the military of the commercial aviation field. They provide a convenient mean of coping with time zone change. Certain watches and chronographs do simply display time on a 24 hour scale, some combine both 12 and 24 hour reading, and others combine two different 12 hour displays. For detailed info about all the different sorts of watches, refer to Multi-Time Zone Watches, written by Phil Baker.

In the nineteen fifties, Rolex designed the GMT Master, which was the very first watch to feature a dual time complication showing home time on a 24 hour display and local time on an independent 12 hour display. The feature proved out to be very convenient and allowed the pilots of PanAM to keep track of their home time while flying across time lines. Aviation professionals and licenced amateur radio hobbyists still use the 24 hour display for Zulu, GMT or UTC time and the 12 hour display for local time.
In the meantime, established watchmakers such as Seiko, Omega and Blancpain have adopted this display, making it the standard for dual-time wristwatches.

This page aims at explaining the benefits of the 24 hour watch with independent 12 hour reading. For further explanations on "daylight saving" and a time zone chart, see the page Daylight Savings or "Summer" time... from Chuck Maddox

ANATOMY OF A GMT WATCH

(1). SECOND HAND

(2). MINUTE HAND

(3). INDEPENDENT 12 HOUR HAND: like a conventional watch, displays AM and PM local time

(4). 24 HOUR HAND: usually arrow-shaped and red, displays 24 hour home time

(5). DATE WINDOW

(6). SETTING CROWN, three positions: screwed in/out, setting position 1, setting position 2

(7). BIDIRECTIONNAL ROTATING BEZEL: when present, either features 24 hour markings, 360 degrees bearing, 60 minutes divisions or major cities names

The watch pictured left is the GMT from Blancpain's Trilogie, a remarkable shot by fellow watch enthusiast Mike Margolis

Similar watches are

-Without bezel: Rolex Oyster Perpetual Exporer II
-With bezel: Omega Seamaster GMT, Rolex Oyster Perpetual GMT-Master I, Rolex Oyster Perpetual GMT-Master II, Seiko Kinetic/AGS Flightmaster, Seiko Kinetic/AGS Landmaster, Seiko Kinetic/AGS Landmaster Southpole, Seiko Kinetic/AGS Landmaster Sagarmatha, Seiko Kinetic/AGS Pipin Ferreras, Seiko Kinetic/AGS Scubamaster, Seiko perpetual calendar

READING A GMT WATCH

This is the diagram we're going to use. The minutes hand, the 12 hour hand, the 24 hour hand and the date window are represented.
(The second hand has been left out to keep the dial less busy)

Like on a conventional watch, the 12 hour hand rotates twice a day, displaying AM and PM time on the dial.

On a conventional watch, the hand would be showing 10:10. But one would not be able to tell if it were AM or PM time

The 24 hour hand rotates just once a day, pointing at the outer ring, or bezel. And by doing so, it helps to differentiate between AM and PM time.

Here, the 24 hand points at 10:10 (10:10 AM), and not 22:10 (10:10 PM)

Reading the display might turn out to be quite tricky at the beginning, yet once one gets the hold of it, it becomes a child's play:

As one can see, the 24 hour hand completes a single revolution, whilst the 12 hour hand rotates twice, successively displaying AM and PM time.

SETTING A GMT WATCH

Here lies the particularity of the independent 12 hour complication first used by Rolex, and now Blancpain, Omega and Seiko:

Pulled in setting position 1, the crown makes the 12 hour hand jump forward or backward without affecting the rest of the mechanism.

When crossing a time zone, the wearer can simply adjust the 12 hour hand to local time, whilst the 24 hour retains home time.

The date indication is also for local time, so it only changes when the 12 hour hand has completed two revolutions

Pulled in setting position 2, the crown moves the whole 3 hands forward or backward and hacks the second (except on the Blancpain, where the second doesn't hack)

TIPS

Using a GMT watch like a professional

It has been brought to my attention that aviation professionals and licenced amateur radio hobbyists use their 24 hour hand to display UTC time, or Zulu Time:

They set the whole watch on Greenwich Mean Time (GMT 0), and then set the independent 12 hour hand on local time

In our example, if it is 10:10 AM in New York (GMT -5), the whole watch must be set 5 hours forward to display the Greenwich Mean Time (GMT 0)
The independent 12 hour hand can then be set 5 increments backwards to display local New York time (GMT -5)

After this operation, the independent 12 hour hand is no longer synchronized with the 24 hour hand, unless it is being set 5 increments forward

Using a third timezone

It is possible to use a rotating bezel with 24 hour subdivisions to indicate a third time zone:

Simply rotate the bezel by one increment, per hour of time zone.

- Rotate the bezel counter clockwise if the third time zone is Eastern to home time
- Rotate the bezel clockwise if the third time zone is Western to home time

Example: if home time is New York (GMT -5), and third time Paris (GMT+1), move it 6 increments counter clockwise (+6)
If home time is New York (GMT -5), and third time San Francisco (GMT- What is the deal with GMT? Icon_cool

, move it 3 increments clockwise (-3)

Using the GMT display as a rough solar compass

This tip also applies to practically all the other watches equipped with a 24 hour hand. The wearer can use a 24h dial for approximate navigation anywhere between the Tropic of Cancer and the Arctic Polar Circle:

Important! Both the 12 and the 24 hour hand must be synchronized to display local time

Method 1: laid horizontally, the dial will head North if the 24 hour hand is pointed towards the sun

Method 2: if the 12 hour hand is pointed at the sun, the 24 hour hand will point North

The margin of error directly depends on the difference between local official time and actual local solar time. But one doesn't need to get too much in the technical details for approximate urban navigation

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M A R K

Subject: Re: What is the deal with GMT? Wed Apr 14, 2010 7:36 pm

Fascinating, as well as VERY informative !!! I don't have a watch
with GMT complication, but this is some excellent information on
how it all works. What is the deal with GMT? Icon_smile