Emulator

https://www.reddit.com/r/Gamecube/comments/i772nv/another_obscure_gamecube_item_arrived_today/
https://maru-chang.com/hard/dol/index.htm
per deepl
A peripheral device that apparently allows you to count down to the release date of a game by plugging it into the GameCube unit's memory card slot in the Nintendo Monthly in-store demo. It has been implemented since the December 2004 issue of Nintendo Monthly, but details on what this calendar card allows you to do (and the calendar function itself is on the GC itself) have not been revealed. We are looking for more information. The bulletin board is here. It looks similar to the plug unit part of the GC microphone (DOL-022) with the hole for the cable to exit sealed, and appears to have been diverted. Naturally, it is not for sale to the general public.