Since manufacturing his first timepiece, Hans Wilsdorf was steadfastly on the pursuit of reliable and accurate wristwatches.  Early Rolex innovations such as the automatic winding movement and the Oyster waterproof case are the foundations for an accurate wristwatch. Before the atomic age, defining and measuring accuracy was left to the observatories who tracked the earth’s movement in relation to the stars overhead. To debunk the myth that wristwatches were not accurate relegated to second class citizens in a world surrounded by pocket watches, Rolex  blew the inaccuracy myth by being the first wristwatch to earn chronograph certification by the Kew Observatory in England in 1914, a distinction typically earned by marine chronometers at the time. Since the early 20th Century, Rolex always pursued third party chronometer certification to keep the accuracy naysayers at bay.


Fast forward to 1972 at a time when other wristwatch manufacturers claimed their watches to be accurate without any reference to a standard, the Contrôle Officiel Suisse des Chronomètres (COSC) was formed in 1973 to certify watch movements for both accuracy and durability. COSC certification is only for the movement, not the entire watch to keep the testing focused on movement accuracy and reliability and to keep brand names out of influencing any result.  Movements are tested with the winding balance and watch face removed and over the course of 14 days in a variety of temperatures and positions.  To earn COSC Chronometer Certification, a watch movement has to run within these parameters:

Average daily rate: -4/+6 seconds / day

            Mean variation in rates: 2 seconds / day

            Greatest variation in rates: 5 seconds / day

Difference between rates in Horizontal & Vertical positions: -6/+8 seconds / day

            Largest variation in rates: 10 seconds / day

           Thermal variation: ±0.6/p>

           Rate resumption: ±5 seconds /day



Once passing 14 days of testing, a watch movement is Officially Certified Chronometer and only then can a Officially Certified Chronometer designation appear on the watch face. Every COSC chronometer has a serial number engraved on the movement and a certification number provided by COSC.




Rolex has led the industry in the number of COSC certifications awarded each year with Omega and Breitling holding their traditional places of second and third, and the latest numbers from 2013 are no different:
Omega 477,477
Breitling 155,737


Rolex slightly surpassed the 800,000 movement mark, increasing its certified chronometer movement production by 10% since 2008, the year the Great Recession hit. 800,000 certified chronometer movements are a product of approximately 3,200 certifications a day.




Most of the watch models in the Rolex line have earned COSC Chronometer Certification, with only a few models not submitted for testing.  Rolex watches have a self-imposed stricter standard than COSC of -1 to +5 seconds per day.  The amount of mechanical precision required for a movement to stay within COSC specifications is mind-boggling.  With 86,400 seconds in a 24 hour period (31,536,000 sec/year), a difference of +3 seconds per 24 hours is a deviation of 0.000035 (thirty five millionths) of its daily function that converts to being 99.99% accurate. A Rolex watch balance oscillating at a frequency of 28,800 times per hour, is an equivalent to a car traveling 87 mph and over a distance of 3,600 miles a year (every hour the watch in on your wrist).   It is comforting to know that the highly precise mechanical Rolex on your wrist is certified by COSC and carries a chronometer certification that most other brands do not have, or care to have.

-Sheldon Smith