they come out with new snell ratings every 5 years, and a helmet is good for two of them( 10 years)
So if you bought a 2010 snell, it expires in 2020, etc, etc.
It pays, regards helmets to pay attention when you buy them what snell they have. I bought a 2015 about 4 years ago, it’s good to 2025, so I will get about 7 or 8 years out of it. The 2020’s hadn’t come out.
Frankly, it sucks, but whoever sold that one to you should have clued you in it was set to expire
Right now, 2015 and 2020’s are good to go.