It's probably not on the label. It may be stamped on the cryo, but not likely. It is on the case that the briskets came in so someone at RD would have to get that info for you.
I wouldn't feel comfortable with 40 days without knowing the date. For example, I bought a case of CAB briskets and they were already at 30 days when I got them. Without knowing the date you can't be sure.
One "rule of thumb" that may help is to look at the cryo. Assuming that it hasn't been breached if you see bubbled in the cryo that are about dime sized then the brisket is about 30 days aged, nickel sized would be 40 days and quarter sized would be 50 days, but again, without knowing for sure I wouldn't take a chance.
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