Here's why:

* Chronometer Certification: Rolex Submariners (both date and non-date) are certified chronometers by the COSC (Contrôle Officiel Suisse des Chronomètres). This means they have passed rigorous testing for accuracy and precision.

* COSC Standards: To receive chronometer certification, a watch must meet specific standards:

* Accuracy: The watch must maintain a timekeeping accuracy of -4 to +6 seconds per day when tested in various positions and temperatures.

* Power Reserve: It must have a minimum power reserve of 48 hours.

* Other Tests: The watch also undergoes tests for shock resistance, magnetism, and waterproofness.

Therefore, a Rolex Submariner (non-date) is a certified chronometer, meaning it's highly accurate and reliable.