Hmm Im not certain c02 bottles have a test date, its been a while!
@tommikka
I'm going to say I can't remember either?
I'm sure I have a dated co2 cylinder, but small ones can be exempt anyway (similar to the American 2 x 2 rule)
Either way for the price of a new co2 cylinder it's cheaper to buy then test
If the cylinder has no date markings then there's no born or expiry date.
Any markings would be stamped in the neck