In the absence of a year of expiration, I think context would demand that it would be the first July 31 after the coupon was delivered, as a matter of giving a reasonable interpretation to a written instrument.
The main reason that coupons have expiration dates has to do with accrual accounting, with the actual liability arising from having to honor old coupons without an expiration date as a secondary consideration. Coupons are listed as contingent liabilities of firms with accrual based financial statements (including accrual based taxpayers) in an amount equal to a reasonably determined estimate of the number of coupons that will be used before they expire. Then, when they expire, the contingent liability is removed from the books. If you issue a coupon without an expiration date, then you have to leave some proportion of the contingent liability on the books forever, reducing it slightly each year based upon a reasonable estimate of the likelihood of the coupons being redeemed. Do this enough times and it becomes an accounting nightmare.
There is certainly a credible legal argument that an unexpired coupon has to be honored. If the coupon is treated as a contract (which is questionable due to lack of consideration, but it is possibly still enforceable if relied upon via promissory estoppel although proving reliance on a coupon used long after it was issued may be hard), the statute of limitations runs from the date that the coupon is dishonored and the contract or promise is breached.
But, the dollar amount to litigate if it was denied is usually so small (and there is no right to attorneys fees in a dispute of this kind in most U.S. jurisdictions), that almost nobody would every bring a lawsuit over it. So the fear of actual dollar liability from old coupons is minimal in most cases. Also, old coupons for fixed dollar amounts off erode in value with inflation.
If there is a misprint in the coupon and it is much larger than intended, than good faith mistake will often excuse a firm from having to honor it, as the question notes.
This analysis does not consider individual state consumer protection statutes. These statutes vary considerably from state to state but sometimes declare that failure to honor a coupon is a deceptive trade practice, akin to false advertising.
These claims would often run from the time that the coupon was issued to the first time that one was dishonored, rather than from each time that a coupon was dishonored. But, consumer protection laws often add a minimum statutory damages award per violation, allow for awards of attorneys fees to a prevailing consumer, and allow for class action lawsuits and/or lawsuits by the state attorney general or local DAs on behalf of consumers. This could make the case economically significant.
Even then, determining who is part of the class of people harmed by a firm's failure to honor old coupons without an expressly stated expiration date would be hard because you'd have to identify people who (1) made a purchase that the coupon could have applied to, (2) still had the coupon at the time the purchase was made, and (3) had it dishonored in the discretion of the firm issuing it. Most people who receive coupons never purchase the goods advertised and don't use the unexpired coupons that they have when they do make purchases covered by the coupons. These considerations disfavor the idea that a class of people harmed by the practice of dishonoring old coupons with no expiration date could be formed at all, since there would be key factual distinctions between people whose coupons were actually dishonored and those whose coupons were not dishonored.
The economic purpose of coupons as opposed to simply reducing the price at the store, is to engage in "price discrimination" that causes people who would only buy at a lower price to make a purchase, while allowing the firm to charge the full price without a coupon for people who don't care enough to locate the coupons and use them at the time of the purchase. Thus, the store keeps the sales that would have happened anyway, but gets additional sales from coupon users that it wouldn't have otherwise made. So, an award of the full amount of unexpired coupons issued and not exercised would be grossly too high.
One could also imagine a lawsuit for discrimination by a place of public accommodation, if coupons were sometimes honored and sometimes dishonored, based upon a protected class under those discrimination statutes.