Yesterday (20/04/2023) I purchased a 16-25 Railcard from LNER. It was advertised as valid for "1 year" on LNER's website. When I added it to my device, it had an expiry of 19th April 2024.
Presumably the (undocumented!) logic here is 365 day validity, 2024 being a leap year. I believe the spec...