When you set an expiration date for a coupon, our system will understand that once that day is complete, the coupon should not be redeemable. So the coupons will expire exactly at midnight at the end of the day you choose.
For example, if I want to offer a New Year's Day coupon that users can redeem on January 1st, then you will want to set the expiration date to January 1. So that when midnight comes rolling, as soon as it is January 2nd, users will no longer be able to redeem the coupon. That will give users the full first day of January to redeem the code.
If you do not set an expiration date, then the coupon will never expire. It can only stop being redeemable if you have added a redemption limit to the coupon code.
To learn how to add an expiration date to a coupon or a redemption limit, head over to our Create Coupons article.