When I use a /tan command in a command block (for example, /tan setseason mid_winter), it outputs "You must specify which player you wish to perform this action on" even though player selection is not relevant to the command.

Seasons are saved per-world ; and the commands code (example here for setseason) is using the CommandSender to retrieve the world that should be updated, thus making it unusable from a command block (and maybe from the console too).