Most people set a watch once, notice a few minutes later that it disagrees with their phone, and shrug. If you care about accuracy, whether you own a mechanical heirloom or a quartz diver, there is a better method. It takes about a minute to set a watch precisely, and about a week to learn how well it actually keeps time.
A reference clock you can trust, a watch, and a notebook or notes app. For the reference, use a clock that is corrected against a network time source instead of relying on your own device's unsynchronised clock. Our live clock displays the current time down to the second. Be realistic about its limits: any web page is affected by network delay and by how your browser handles timing, so treat it as accurate to a fraction of a second. That is plenty for setting a watch by eye, because human reaction time is around a fifth of a second anyway.
If your watch has a "hacking" movement, the seconds hand stops when you pull the crown out. That lets you do this properly:
If your watch does not hack, set it a few seconds ahead of the reference and wait, or accept an error of a second or two. Many older mechanical watches and some cheaper quartz models don't stop the seconds hand.
Write down the date, the time, and the offset between the watch and the reference. Right after a careful setting it should be close to zero. If you estimate the offset by eye, note that you can realistically read it to about half a second.
Compare the watch with the reference at the same time the next day. Write down how many seconds fast (+) or slow (−) it is. This single number is your daily rate, and it tells you most of what you need to know.
One day of data can mislead you. A watch can run differently after you wear it, take it off, or change your activity. Log the offset once a day for seven days. Then divide the total change in offset by the number of days.
For example, if the watch is 2 seconds fast on day one and 9 seconds fast on day eight, it gains roughly one second per day. A steady rate like that is good news, because a consistent error can often be corrected, while an erratic one points to a problem.
Here are rough expectations. Manufacturers and individual watches vary a lot.
| Watch type | Typical accuracy |
|---|---|
| Basic quartz | around ±15 seconds per month |
| High-accuracy quartz | around ±10 seconds per year |
| COSC-certified mechanical chronometer | −4 to +6 seconds per day (test standard) |
| Ordinary mechanical | anywhere from a few seconds to a minute per day |
Your own numbers will also change with temperature and how you wear the watch.
Quartz watches are usually consistent but slightly off, and most can't be adjusted at home. Reset the time when it bothers you. Some higher-end models have an inhibition or rate-adjustment feature described in the manual.
Mechanical watches have a regulator, and a watchmaker adjusts it. Turn it yourself only if you know your movement, because a small nudge changes the rate more than you'd expect. Use your logged rate to tell the watchmaker whether the watch runs fast or slow, and in which position you leave it overnight.
If you travel, set the time by referencing the local clock of your destination. You can check the current time and UTC offset for cities like London, New York, Tokyo or Sydney on their pages before you go.
A watch that loses one second a day is off by half a minute in a month, so a five-minute weekly habit is enough to keep it honest. For the science behind the reference itself, read our guide to how atomic clocks and UTC work.