clear all carts at midnight

You would need a cron job to run a MySQL query to empty both the hikashop_cart and the hikashop_cart_product tables.
stackoverflow.com/questions/21196613/run...-query-as-a-cron-job

Hello,

The HikaShop cron system is there to call your website with a specific period but it cannot assure you that your cron-job will be called a exactly midnight.
It would be better for you to create a specific cron entry via your hosting so it will be executed at midnight (and only once a day).
Otherwise, you can do things thanks to custom plugin but you'll need to introduce checks and "securities" in order to be sure to clean your carts once a day, etc.

Regards,