Setting the Rolex GMT Master II

Rolex "GMT Master" watches have been around since the late 1950s. Steady incremental improvements have been added over time, and this watch has even been to the moon! It is very popular among professional soldiers and commercial airline pilots. It is my opinion that this is the coolest (mass-produced) Rolex ever made, because it has four hands, and not just the usual three, along with a bezel dial that you can rotate! In other words, there are five things that you can play with when you set this watch:

(from http://www.rolex.com)

Let us label everything as follows:

Everybody knows how to set a regular watch, with S, M and H. But T and B confuse nearly everybody. The factory manual that I read (perhaps there are newer revisions?) does not describe the correct convention to set T and B. In fact, the manual is very vague and leaves everything up to the taste of the reader. Indeed, every single person that I have seen wearing one to date has missed the point of how it should be set, as hard as that is to believe, and I fear that they're wearing it for petty cosmetic reasons, as gaudy jewelry, and not because they grasp its great utility. (But who am I to question the pursuit of cosmetic excess? :)

A small amount of thought reveals that there is only one correct convention to set this watch. Under this convention, you can look at this watch's face and immediately report the following (click the image to enlarge):

This watch is in PDT,
GMT+(-7), as B = -7.

It is 18:54:09 (6:54:09pm) in my time zone. I am currently seven hours behind GMT---this means that I am either in Arizona, California during the summer, or parts of Mexico. (I'm actually in California, so it is 18:54:09PDT.) It is 01:54:09GMT.

Then, if you asked me what time it was in Tokyo, for example, I'd know that it is (probably) 10:54:09 (10:54:09am). Greece? (Probably) 03:54:09 (3:54:09am). This kind of calculation is very easy, but you have to set your watch properly, and understand a few simple facts, outlined below.

GMT, Standard Time, and Daylight Savings Time

Before turning to issues specific to the watch, it is important to understand the idea behind GMT, or "Greenwich Mean Time." Greenwich is in England. (We really want "Universal Time" [UT], but for our purposes it is the same as GMT.) GMT is an international concept, and the watch was named after it.

GMT is the same everywhere in the world; it does not depend on your location. If you ask any two people what GMT is, at the same moment, they should give you the identical answer. GMT does not change over time zones, and it does not have "daylight savings time" (DST) corrections, etc.

Once you know what GMT is, you can figure out the "standard time" for a specific country (or portion thereof, in some cases) of interest. Once you know the "standard time" in said country, you can take "daylight savings time" (DST) into account, if needed (should it be summer), and finally get the correct local time, i.e., the time that a native would report if asked in the street.

So here is the recipe: GMT --> standard time --> local time (includes DST corrections).

To determine the standard time, you add to GMT as follows, where the integers represent hours:

CityStandard Time = GMT +Local Summer DST Offset
Hawaii-10None
Los Angeles-8+1
Mexico City-6+1
New York City-5+1
Buenos Aires-3None
*LONDON*01
Paris+1+2
Rome+1+2
Athens+21
Moscow+3??
Baghdad+3+3
Tehran+3.5None
Bombay+5.5None
Taipei+8None
Tokyo+9None
Sydney+10None

WARNING---There are undoubtedly mistakes here, so be careful to lean the answers for your favorite countries.

There are some excellent resources on the Internet that give more cities, where some are mentioned in the reference section of this document, and I also have an extended version of the above table.

It is evident that you can make good guesses if you think about geography for a few minutes, but you need to be a lawyer to be sure. (Consult a good map if you're curious; the lines for the time zones make all kinds of crazy twists.)

Adjusting for daylight savings time is a bit of a problem. If the country is around the equator, don't add anything. If the country is enjoying summer, add zero or +1 hour in most cases. Note that summer in the northern hemisphere is winter in the southern hemisphere, and the converse holds; so when it is winter in South Africa it should be summer in Greece.

Example: Los Angeles, Greece and South Africa

The local time in Los Angeles is GMT+(-8) in the winter (i.e., "Pacific Standard Time"), or GMT+(-7) in the summer (we add +1 for DST, to get "Pacific Daylight Time"). Summer in Greece is the same as summer in California, as they're in the same (northern) hemisphere, so the time in Greece should be about GMT+2+1, or GMT+3. South Africa is probably stuck in winter at this time, so they probably use standard time, so it is probably just GMT+2.

Interpreting the GMT Master's Face

The watch's movement drives S, M, H, and T, but only you can move B. In the following figure, S = 09, M = 54, H = 6; that's clear. What about T? Read T against B. T seems to be something like T=18.9. So it is nearly 19:00:00, but not just yet! We stick in the values for M and S, so it is 18:54:09 in our time zone (nearly 19:00:00).

This watch is in PDT,
GMT+(-7), as B = -7.

But what time zone are we in? You also can note that B's triangular marker is seven steps to the right (clockwise), so B=-7. If the marker is turned to the left (counter-clockwise), B>0. B=-7 suggests Arizona in the winter, California in the summer, etc.

As the picture was taken in California in the (local) summer, adding +1 to the time is probably going to be correct, at least for locations in the northern hemisphere. I know that South Africa is GMT+2 (standard time), and it is winter there, so we're GMT+2, or (2+01):54:09, or 03:54:09. Greece is GMT+2 for standard time, and they probably add an hour in the summer, making it (2+1+01):54:09, or 04:54:09.

It is hard to remember countries that add an extra half-an-hour, and exactly when places start DST, but at least you'll be close.

You can figure out the M and S in GMT from reading the hands. You could move B=0 and read the time directly from T, M and S. But to understand the hour, there is an easy shortcut: read the T hand on the inside dial using the hour markers for H (it reads about 0.9), double that (1.8), round down to the lowest integer (1), and you have the hour. So it is 01:M:S, or 01:54:09. In other words, you don't have to change B to figure out the time in GMT, you can do it instantly.

Setting the GMT Master II

The winding stem has four positions.

How to set B

You don't have to touch the stem to play with the bezel. If your local time zone is negative (you're west of Greenwich), rotate B clockwise by the correct number of hours. If your local time zone is positive (you're east of Greenwich), rotate B counterclockwise by the correct number of hours. If done correctly, T, when read against B, shows the twenty-four hour time in your time zone.

How to set H

When you travel somewhere, set B to the correct time zone. To change H, select position two, and adjust H until the hour is correct (in +/- one hour increments). Note that M, S and T will continue to turn, so your watch will not lose time! Restore position zero when finished.

How to set T and M (and Stop S)

Move B so the arrow is above the crown (B=0). Select position three and adjust T to give the 24h time in GMT, where T is read against B's scale. You'll note that M moves when you rotate the stem, but S stays fixed. (S and M run at every position except three.) When done, restore position zero.

If you don't know what time it is in GMT, ask the United States Navy, a Global Positioning System (GPS) unit, etc. For our purposes, GMT is the same as Universal Time, although there are minor differences outside the scope of this article. (Select position three and turn the stem until you get the correct time.)

Once you set T, you will never have to change it again, except to adjust for error in the watch---T does not change with daylight savings time, for example.

If the watch stops for a few hours?

In the event that the watch stops for a few hours, pull the stem all the way out (Position 3) and turn the M, H and T hands around until the correct time is reestablished. If you should jump the hour hand forward (Position 2), then T will be set incorrectly.

How does one skip the date forward?

Put the GMT Master II in Position 2 and it is easy to skip forward as many days as necessary, one hour at a time. Roughly every other month one has to skip forward a few days because the watch does not adjust for months shorter than 31 days, but this is an extremely quick adjustment.

What if you have a GMT Master I?

This does not have the jump-adjustable hour hand, and I'm not sure as to how one sets it in detail. However, T and B somehow be set to the correct convention, so everything should be fine.

The Seiko SLT009P and SUZ005P

The Seiko SLT009P and SUZ005P

I am pleased to report that Seiko seems to have made a GMT Master II clone, the SLT009P (above). This Seiko seems to be relatively inexpensive and extremely accurate (20s/year), so it might be worthy of consideration!

Further Reading

See The United States Naval Observatory and Markus Kuhn's summary on date and time notation for some excellent references. You should also employ a search engine; looking for "time standards" or "time zones."

iSBiSTER sells some nice time-zone software for PCs running Windows.

An entertaining book on the history of Rolex is The Best of Time, ROLEX Wristwatches, An Unauthorized History, by James Dowling and Jeffrey Hess (ISBN 0-7643-0011-3.)

Don't forget Rolex's web site.

Kleanthes Koniaris, email.