Want to know what time the sun sets in Istanbul? Consult a Geochron

''For tribal man, space was the uncontrollable mystery. For technological man , it is time that occupies the same role,'' argued Marshall McLuhan, the late futurist and media guru.

He must never have encountered a timepiece called the Geochron. During the last decade this device, possibly the world's most elaborate descendent of the sundial, has been taking much of the mystery out of time.

The Geochron is the inspiration of a small, almost elfin inventor named James Kilburg. Operating out of a factory here, he and his workers make two dozen to four dozen of these elaborate timepieces each month for shipment to every part of the globe.

Ronald Reagan took a Geochron with him to the White House. Henry Kissinger was a recent customer, after seeing one in the office of Secretary of State George P. Shultz. The National Aeronautics and Space Administration uses one in its satellite-tracking operations. Geochrons grace a number of corporate board rooms. They even have appeared in a few movies and television shows.

Perhaps the best description of this unusual device comes from Seoul. A Korean wrote the factory asking for information on the ''Day-and-Night-Wall-Map-Clock.''

The Geochron, measuring about 2 feet by 3 feet, is an illuminated map of the world with time zones clearly marked on it. The map slowly moves - one inch every hour - and a scale at the top shows the current time anywhere in the world. It also displays the day and date for different parts of the globe. But its most fascinating feature is that it shows which areas of the Earth are in daylight and which are in darkness.

The inspiration for the Geochron dates back to 1962, when Mr. Kilburg was visiting relatives in Luxembourg. ''The phone rang in the middle of the night. It was my wife calling from the United States. The phone company had given her the wrong information about when to call,'' the silver-haired mechanical engineer recalls. ''I thought, there must be a better way to give people this information.''

About this time, Kilburg left the company where he had been working, so he turned his attention to the time-zone problem. A year later, he had a working model of the Geochron. In another year he was ready to begin production.

''I started the company with my own money. There was no way to know whether it would succeed,'' Kilburg recalls. Since then he has sold thousands of Geochrons in every part of the world and never placed a single advertisement.

The Geochrons are expensive - $955 plus shipping costs - in a world where digital watches go for as little as $15. Frequently, customers are institutions with multinational business - who often need to know the time anywhere in the world.

People who are just curious about the physics and politics of time also can learn a number of things from Geochrons. For instance, most people imagine that time zones are spaced evenly around the world. In fact, time-zone boundaries zigzag like the course of sailboat in a head wind. The continental United States , for instance, is divided into four fairly even time zones. But China, roughly the same size as the US, is one zone.

The Chinese adjust their schedules rather than time, Kilburg says. In one part of China, the sun might rise at 5 a.m. In another, it rises at 9 a.m. ''I understand their system works very well. It makes you wonder whether all our time zones are really necessary, or if there isn't a simpler way,'' he muses.

By manually moving a Geochron through a year's cycle, the date of the longest and shortest days of the year can be determined for any location. A moving dot depicts the point on the Earth's surface that is directly under the Sun. As the dot moves around the world, it describes a figure-eight path called an analemma, a figure used in sundials of the ancient Greeks.

''I've been a mechanical engineer all my life and have been involved in many projects. But none has ever given me nearly as much satisfaction,'' Kilburg says.