The Rolex Superlative Chronometer certification COSC has been a mainstay for Rolex since the mid 1950's.  Rolex guarantees your watch is accurate to -2/+2 seconds per day, after casing. The COSC requires only a -4/+6 per day accuracy.  Even Omega's Master Certification requires a 0/+5 accuracy. 

Rolex considers accuracy the utmost importance to its timepieces and strives to achieve near perfection in its craft - demanding of itself higher standards than the competition.  

Rolex runs each of its timepieces through a battery of tests internally.  Besides precision, Rolex tests waterproofness, power reserve, and self-winding.  Once completed, these movements are sent out to COSC for an official independent certification, before casing.  

The Green Seal that comes with a Rolex includes a guarantee and warrantee of 5 years.  The green seal means that Rolex has spent tremendous amount of money and time to ensure each timepiece is accurate and reliable.  On many of the timepieces Rolex proudly displays the Superlative Chronometer wording on  the dial itself.