Why can’t I choose my departure date in the list for international shipping?

Automated shop site

If the shop won’t let you choose your departure date, then we don’t consider that date to be a safe departure date for the shipping method you’ve selected. If you chose basic international, you can try express and see if your departure date is now allowed. Please do not tell the system a false departure date just to make the order complete! If there is not enough time for an international shipping method, then you’ll have to choose a shipping method within Japan, such as to your hotel or AirBnB.