Accuracy: what's normal, what's not.
Most complaints about a watch running badly turn out to be complaints about a watch running exactly as its specification promised. Knowing the difference will save you a service bill, and it is the whole point of this lesson.
The unit: seconds per day
Mechanical accuracy is quoted in seconds per day, usually as a range with a sign. −10/+20 s/day means the watch may lose up to ten seconds or gain up to twenty in twenty-four hours.
The sign matters more than the size. Makers deliberately regulate many movements to run slightly fast, on the reasoning that a watch which gains is a watch you are never late by.
Quartz gets quoted per month instead, because otherwise the numbers would be absurd fractions.
What normal looks like
A standard, uncertified mechanical movement of the kind you find under $1,000 is typically specified somewhere in the region of −20 to +40 s/day, and commonly performs better than that once it has settled. Citizen’s Miyota 8203, in the NY0086, is the classic example of a movement people call inaccurate when it is in fact hitting its own published tolerance exactly.
A quartz watch will usually sit within about ±15 seconds a month. A radio-synchronised solar quartz, like the Casio GW-M5610U, corrects itself against a broadcast time signal and is effectively never wrong at all.
Positional error is why a mechanical watch can gain lying dial-up and lose standing on its crown: gravity pulls differently on the balance depending on which way the watch is facing. Which gives you a free trick. Leave a fast-running watch crown-down overnight and you pull its daily average back toward zero, at no cost, using nothing but a bedside table.
Certification, and what it is worth
A chronometer is a movement that has passed independent testing to a defined standard. The best known is COSC in Switzerland, whose mechanical standard is a mean daily rate of −4 to +6 s/day across a multi-day test in several positions and temperatures. Various makers run their own in-house standards, some of them stricter.
Certification is real and it is not free. But it certifies the movement, not the finished watch, and it tells you nothing about the case, the bracelet, or whether you will enjoy wearing the thing at all. Treat it as one line on the spec sheet rather than as a verdict, and be suspicious of any brand that treats it as the headline.
When to worry
Worry when the rate changes suddenly, when the watch stops well short of its stated power reserve, or when it gains or loses in minutes rather than seconds. Those are symptoms of a movement wanting service, a magnetised hairspring, or damage.
A watch that has quietly gained twelve seconds a day since you bought it is not broken. It is a watch. A watchmaker will usually regulate it closer for a modest fee, and you will feel silly for worrying.