The Fascinating History Behind World Time And Precision Watches

Luxury Watch Culture • Precision Engineering • Global Timekeeping

III. Common Timekeeping Terms

(I) World Time (Greenwich Mean Time)

World Time is also known as Greenwich Mean Time (GMT). At the International Prime Meridian Conference held in Washington D.C. in 1884, it was decided that the meridian passing through the Royal Observatory in Greenwich, England, would be designated as the Prime Meridian — also called the Principal Meridian. The zero time zone, whose local time is based on this meridian, was established as the world standard time.

Note: Greenwich Mean Time represents 12:00 noon; the International Date Line represents 24:00 midnight.

(II) The International Date Line

Commonly abbreviated as the Date Line (Figure 1–1–14).

Once World Time and the global time zone system were established, a new question arose: where exactly does a new day begin on Earth?

Before the Date Line was established, a confusing problem frequently occurred. For example: when it is Monday 20:00 in Beijing, calculating eastward by the time zone system, East Time Zone 12 would show Monday 24:00; calculating westward, West Time Zone 12 would show Tuesday 0:00. So at the very same moment, is it Monday 24:00 or Tuesday 0:00 in the same time zone?

To resolve this dilemma, an international conference was convened, and it was ultimately decided to designate a line near the 180° meridian as the International Date Line. To avoid dividing certain island groups, the Date Line is not a straight line — it deviates around land masses. With the exception of Antarctica, the Date Line does not pass through any continental territory.

Every new day on Earth begins at the Date Line — making the Date Line the earliest place on Earth for each new day.

The 1884 international conference also established the rules for date changes at the Date Line:

• When crossing the Date Line from west to east, subtract one day (e.g., crossing on June 8 means the date reverts to June 7; the following day is still June 8).
• When crossing the Date Line from east to west, add one day (e.g., crossing on August 9 means the date advances to August 10; the following day becomes August 11).

With the Date Line established, the earlier confusion was resolved. As long as everyone follows this convention, dates and days of the week around the world will never fall into chaos.

(III) Beijing Time

China's capital, Beijing, is located at 116°E longitude, which falls within East Time Zone 8. However, the central meridian of East Time Zone 8 is 120°E — so "Beijing Time" is technically the local time of 120°E longitude, a point approximately 340 kilometers east of Beijing.

China's territory spans 60 degrees of longitude from east to west — equivalent to 4 time zones. However, for the sake of convenience and national unity, China uses a single time zone throughout the entire country: East Time Zone 8, known as "Beijing Time," serves as the unified national standard time.

It is worth noting that the world's time zone boundaries do not strictly follow lines of longitude. For practical reasons, countries often draw time zone boundaries along administrative or natural geographic lines. The United States, for example, uses 4 time zones.

(IV) Leap Seconds

The leap second was born after the international atomic second standard came into effect in 1972. Its purpose is to compensate for the slight discrepancy between the scientific atomic time standard and the actual rotational speed of the Earth. Approximately every few years, one second is added to or subtracted from the standard time, so that scientific atomic time and Earth's rotational time remain aligned.

Leap seconds are generally inserted during the final minute of June 30 or December 31 of a given year. The specific timing of each leap second is announced globally by international organizations.

[News] The Chinese Academy of Sciences National Time Service Center announced that China synchronized a leap second adjustment at 7:59:59 Beijing Time on July 1, 2012. Clocks briefly displayed the unusual reading of 7:59:60. The director of the Beijing Astronomical Observatory explained that the leap second adjustment is necessary because of the tiny difference between atomic time and World Time. When the discrepancy between the two time systems exceeds 0.5 seconds, one second must be added on June 30 or December 31. Leap second adjustments have no impact on the daily lives of ordinary people.

(V) The World Clock

The world clock (Figure 1–1–15) is designed according to the principles of the time zone system. Many modern clocks have incorporated a world time function: a 24-hour disc rotates clockwise, driven by the movement at a gear ratio of 1:2 — the hour hand completes 2 rotations while the 24-hour disc completes just 1. This makes for a relatively simple yet elegant world clock mechanism.

The outermost ring of the world clock dial is a city ring, divided into 24 equal sections in time zone order, with 1–2 major cities marked in each section. The city ring itself does not rotate — only the time display rotates. Once you set Beijing Time correctly, the times for all other cities around the world fall into place automatically.

The 24 Cities of the World Clock: Names, Time Zones, and Coordinates

The following table lists the 24 reference cities used on world clock dials, as established at the 1884 International Prime Meridian Conference (Table 1–1–3):

Notes: 1. Daylight saving time excluded. 2. Longitude: E = East, W = West; Latitude: S = South, N = North. 3. GMT, TWO TIME ZONE, UTC and similar abbreviations all refer to the second time zone function.

The 24 Nations and Cities at the 1884 International Prime Meridian Conference

The following 24 cities were represented at the 1884 Washington Conference that established the global time zone system. These are the cities that appear on world clock dials to this day — though different watch brands may substitute alternative cities in the same time zone. Patek Philippe, for example, has displayed up to 41 cities on its world time watches.

1. London, United Kingdom — GMT / World Time
2. Trinidad and Tobago
3. Caracas, Venezuela
4. Mexico City, Mexico
5. Los Angeles, USA
6. Honolulu, Hawaii, USA
7. Auckland, New Zealand
8. Sydney, Australia
9. Hong Kong, China
10. Colombo, Sri Lanka
11. Dubai, UAE
12. Cairo, Egypt
13. Azores, Portugal
14. Rio de Janeiro, Brazil
15. New York, USA
16. Denver, USA
17. Anchorage, USA
18. Samoa Island, American Samoa
19. Noumea, New Caledonia
20. Tokyo, Japan
21. Bangkok, Thailand
22. Karachi, Pakistan
23. Moscow, Russia
24. Paris, France

These 24 time zone cities remain largely the same as those seen on world time watches today, though different watch brands may add their own city names or substitute cities in the same time zone. If Beijing Time is set correctly, all other world times fall into place automatically — making the world time watch one of the most practical and elegant complications ever devised (Figure 1–1–16).

Note: 6:30 to 18:30 represents daytime; 18:30 to 6:30 represents nighttime. Pressing the button at the 10 o'clock position shifts the 24-hour disc counterclockwise by 1 hour per press.

The World Time Watch: Where History Meets the Wrist

From the 1884 Washington Conference to the atomic second, from the International Date Line to the leap second — every element of global timekeeping is distilled into the world time watch. It is one of the most intellectually rich and practically useful complications in all of watchmaking.

At Aorawa Time, we celebrate this heritage with every piece we craft. Explore our collections:

• Men’s 42mm Skeleton Automatic Watch — see the movement of time itself through an open dial.
• Men’s Full Diamond Octagonal Luxury Watch — precision and opulence across every time zone.
• Vintage Leather Apple Watch Band — timeless craft for the modern global traveler.