Watch complications explained
What a watch complication is, and how each one works — date, day-date, small seconds, power reserve, moon phase, GMT, chronograph, perpetual calendar and more.
In this guide
In watchmaking, a complication is anything a watch does beyond showing hours, minutes and seconds. A date window is a complication. So is a moon phase, a second time zone or a stopwatch. The name is literal: each one adds more wheels, levers and springs to the movement, and makes it more complicated to design, build and service.
If you're planning to make a watch, complications are the last big decision — and the easiest place to overdo it. This guide explains the common ones, how they work, and which are worth having.
The complications at a glance
| Complication | What it shows | Where it usually sits | Difficulty |
|---|---|---|---|
| Date | Day of the month | Window at 3 or 6 | Simple |
| Day-date | Weekday and date | Window or two windows | Simple |
| Small seconds | Seconds on a subdial | Subdial at 6 (sometimes 9) | Simple |
| Power reserve | How much energy is left | Gauge on the dial | Moderate |
| Moon phase | The phase of the moon | Aperture at 6 or 12 | Moderate |
| GMT | A second time zone | Extra 24-hour hand | Moderate |
| Chronograph | A stopwatch | Subdials and pushers | Complex |
| Annual / perpetual calendar | Date that handles month lengths (and leap years) | Several subdials | Very complex |
| Tourbillon | Nothing — it's a rotating regulator | Visible cage on the dial | Very complex |
| Minute repeater | Chimes the time on demand | Slide on the case | Grand complication |
Date
The most common complication of all. A date wheel with 31 numbers sits under the dial and jumps forward once a night. Because it always counts to 31, it needs correcting by hand after short months — five times a year. Most watches place it in a window at 3 o'clock; 6 o'clock looks more symmetrical.
Tip: on many mechanical watches you shouldn't change the date quickly while the date-change gears are engaged, usually from late evening to early morning. Our guide to setting a mechanical watch explains why.
Day-date
A day-date adds a second disc showing the weekday, often abbreviated ("MON 12"). Both discs flip around midnight. It's handy, and it gives a dial a classic, businesslike look. A watch shows either a date or a day-date — not both.
Small seconds
Instead of a long central seconds hand, a small seconds hand runs in its own subdial, usually at 6 o'clock. It's not an addition so much as a rearrangement: the subdial sits on the fourth wheel of the gear train, which turns exactly once per minute. It's the traditional layout for dress watches and old pocket watches, and it keeps the dial calm.
Power reserve indicator
A mechanical watch runs on a wound mainspring, and a power reserve indicator shows how much energy is left, like a fuel gauge. Inside, a tiny differential compares how much the spring is being wound with how much it's unwinding. It's especially useful on hand-wound watches, so you know when to give the crown a few turns. Learn more in power reserve explained.
Moon phase
A disc painted with two moons turns slowly behind a curved window, showing the moon waxing and waning. The classic version has 59 teeth and advances one tooth a day — two lunar months per turn. It's one of the most romantic complications, and our moon phase guide explains how accurate it is and how to set it.
GMT
A GMT watch has an extra hand that goes around once every 24 hours, pointing to a 24-hour scale on the dial or bezel. Set it to home time or UTC, and you can read two time zones at once. Loved by pilots and frequent travellers. See our GMT watch guide.
Chronograph
A chronograph is a stopwatch built into the watch. One pusher starts and stops it, another resets it, and subdials count elapsed minutes and hours. It adds dozens of parts and is one of the most popular complications on sports watches. Variants include the flyback (resets and restarts with one push) and the split-seconds or rattrapante (times two events at once).
Annual and perpetual calendars
An annual calendar knows which months have 30 or 31 days, so it only needs correcting once a year, at the end of February. A perpetual calendar also knows about leap years and runs correctly until 2100 — which is not a leap year in the Gregorian calendar, so it needs one correction then. Perpetual calendars are made from a clever "program wheel" that turns once every four years.
Tourbillon
Patented by Abraham-Louis Breguet in 1801, a tourbillon puts the balance wheel and escapement in a rotating cage, usually turning once a minute. The idea was to average out the effect of gravity on a pocket watch that sat in the same position all day. On a wristwatch its practical benefit is small, but it's mesmerising to watch and a showcase of skill.
Minute repeater
Slide a lever on the case and a minute repeater chimes the time on tiny gongs: low notes for the hours, a double note for each quarter hour, and high notes for the minutes. It was invented so people could tell the time in the dark before electric light. It's among the hardest complications to make and tune.
How many complications should your watch have?
Fewer than you think. Every complication competes for space on the dial. A good rule is one idea per watch: a dress watch with small seconds, a diver with a date, a travel watch with a GMT. Some combinations physically clash, too — a moon phase and small seconds both like to live at 6 o'clock.
Ask what you'll actually look at every day. A date is useful. A power reserve indicator makes a hand-wound watch easier to live with. A moon phase is pure pleasure, and that's a perfectly good reason.
Add complications in Mecha Watch
In Mecha Watch you can add six of these to your own design and see them working: a date, a day-date, a small seconds subdial at 6, a live power reserve gauge at 9 that drains as your watch runs, a moon phase that shows the real phase of the moon, and a GMT hand on UTC. Date, small seconds and power reserve are free; day-date, moon phase and GMT come with the one-time Mecha Atelier unlock. Chronographs, perpetual calendars, tourbillons and repeaters aren't in the app.
Just like a real movement, some combinations don't fit together, and the builder tells you why. Mecha Watch sounds like make a watch — and choosing complications is the most watchmaker-ish part of making one.
Frequently asked questions
What is a complication on a watch?
Any function beyond showing hours, minutes and seconds — for example a date, moon phase, second time zone or stopwatch. The name comes from the extra parts each one adds to the movement.
Is a date a complication?
Yes. The date is the simplest and most common complication.
What is the most complicated watch complication?
The minute repeater is usually considered the hardest single complication, along with the perpetual calendar and split-seconds chronograph. Watches that combine several are called grand complications.
What does a power reserve indicator do?
It shows how much energy is left in the mainspring, like a fuel gauge, so you know when to wind the watch.
Is a tourbillon worth it?
On a wristwatch, a tourbillon improves accuracy very little. People buy it for the craftsmanship and the pleasure of watching the cage rotate.