A dual-time watch has two hour hands in order to indicate time in two different time zones. This complication is popular with travelers, as it allows the wearer to set a “home” time zone for quick reference. The second hour hand is sometimes referred to as the GMT or UTC hand. The second hour hand usually indicates 24 hours instead of the standard 12. This allows the wearer to reference a time zone that is more than 12 hours difference from the home time zone.
To understand the mechanics of the dual-time complication one must first understand how the motion works function. The motion works are a reducing gear train, driven by the center wheel arbor rotating once per hour. The dial of a watch usually indicates 60 minutes and 12 hours. The cannon pinion is attached to the center wheel arbor, and drives the minute wheel at a ratio of 1:3. The minute wheel then drives the hour wheel at a ratio of 1:4. Combining these two ratios results in a ratio 1:12, which works perfectly for the hour hand. The hour hand is then attached to the hour wheel in order to indicate the hours as desired.
A dual-time watch is different from a standard hours and minutes watch in two ways.
First, the second hour hand must indicate 24 hours. To achieve this, two additional wheels are added to the motion works. A second minute wheel mounted co-axially drives a 24 hour wheel at a ratio of 1:8. Combining the 1:3 ratio of the cannon pinion to minute wheel, and 1:8 of the minute wheel to 24 hour wheel results in a ratio of 1:24, which works perfectly for the 24 hour hand. The second difference in a dual-time watch is that the primary hour hand must be independently adjustable in hour increments.The hour wheel has a post on which the hour hand is fit. This post has a star wheel on it’s outer edge with 12 teeth. Those teeth interface with a detent spring on the hour wheel, which allows it to be adjusted in 12 increments of one hour.
