Daily rate

Daily rate is the quantity a chronometry test reports. COSC, the Swiss body that certifies chronometers, gives as the first of its criteria: "Average Daily Rate: The watch must maintain an average daily rate between -4 and +6 seconds per day." It adds that "the average daily rate must be between -4 and +6 seconds per day over the first 10 days of testing". The sign carries the meaning: a negative figure is a watch losing time, a positive figure one gaining it.

A single rate figure is only part of a test. COSC states that "During this time, the watch is tested in five different positions and at three different temperatures (8°, 23°, and 38°C)" across fifteen days, and its published list of seven criteria also includes mean variation in rates, greatest variation in rates, difference between rates in horizontal and vertical positions, variation in rate depending on temperature, and rate resumption. Quartz watches are assessed under a separate procedure; FH's list of available NIHS standards names "NIHS_93-16_fr: Horlogerie - Procedure d'evaluation de la precision des montres a quartz" (2003, modified 2018).

What this Codex does not do: quote a standard's definition of rate. The underlying standard is ISO 3159:2009, Timekeeping instruments - Wrist-chronometers with spring balance oscillator, edition 2, published December 2009, whose public abstract says it "lays down the definition of the term 'chronometer', describing the categories, the test programme and the acceptable minimum requirements for wrist-chronometers". The standard is paywalled and only that abstract is public, so how it defines and computes rate has not been read here. The minus 4 to plus 6 figure is published as COSC's own statement and is cited to COSC.