Why do the Sliders 96, 55 or 57 have a limited use-by date?

Sliders are made from natural materials which may change over time.
It is advised that slider rubbers are replaced 12 months after the date specified on the certificate. However, they must be discarded 24 months from the date of manufacture. When not in use for testing sliders shall be stored in the dark between 2 and 10 °C.
Standard EN 16165 says a rubber is valid for 24 months from date of certificate. Other standards may have other requirements.