Thermal paste with expiry date?

It means that the compund might not work properly to cool your CPU or GPU. Like many products, age will cause it to deteriorate.

there are basically two kinds of TIM: thermal paste and thermal grease/gel. Thermal paste is like AS5: thick, sticky and gooey. These are meant to maintain their viscosity over years at a time. Thermal grease/gel is meant to runny and easy to spread by adding additional liquid to the compound. However, they are so thin, they will be pushed out of the sides and eventually need to be reapplied. The advantage is that thermal greases do not need a break-in time, and you will notice a difference immediately. however, they are thin because of added fluids like water or oil, which can dry out in the tube. since most computers today (owned by people who would care about TIM ) are owned for less than 2 years, either one is usually ok.

In your case, however, this probably means the viscosity is no longer guaranteed. You should check with them just in case. I don't see any shelf-life information at their website.