I still disagree. If a coupon expires on March 22nd, (for example,) It would be good up until midnight of Mar. 21st. I wouldn't submit a coupon to a store on the date of expiry and expect it to still be good.
As far as online coupon codes go, I've used ones with specific times of expiry, as well, so I'd say it's still not unusual, but we can agree to disagree.

Edgar replies: Lana, it is not a matter of disagreeing, it is just the fact: a coupon that expires on December 31, 2010 is good all day. I can't tell you how many coupons I have used at the last minute on December 31 of any particular year. You have been missing out on a full day of possible coupon use!

From couponing101: "Expiration Date: All coupons have an expiration date. Some have an expiration date that actually says “No expiration date.â€ Stores usually will not accept coupons that do not have expiration dates. You can use the coupon through the date of the expiration date. If the coupon’s expiration date is 10/6/2009, you can use the coupon until 11:59 pm on 10/6/2009."