Seconds Per Day
A watch specification field known as Seconds Per Day, used to describe or compare a measurable or identifying characteristic of a watch.
Also known as: s/day, sec/day, Seconds a day, Daily rate
Definition
Seconds per day is the common unit used to express the daily rate error of a mechanical watch. A figure of +5 s/day means the watch gains approximately five seconds over 24 hours; −5 s/day means it loses five. A tolerance such as −4/+6 s/day describes an allowed range rather than a promise that every individual day will land at the same point. Longer observation gives a better picture of average real-world rate.
Why it matters
It makes accuracy specifications directly comparable, but one day can be noisy; owners assessing a watch should look at average rate over several days and normal wearing conditions.