My coupon stopped working a day earlier than the Valid until date. Why?

Valid until takes a date only, and the coupon stops at the very start of that day, not at the end of it. So a coupon with Valid until set to the 31st works up to the end of the 30th.

To keep a code working for the whole last day:

1. Go to Marketing → Coupons and click Edit on the coupon.
2. Set Valid until to the day after your last offer day.
3. Click Save coupon.

Valid from works the same way: the coupon starts at the beginning of that day.