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The sell by date typically is meant more for the store usage as to how long they can present the product for sale.
The USDA website states that beef should be cooked or frozen with 3 to 5 days after the purchase date. If you are outside these guidelines then it is best to discard the meat. If you have refrigerated and handled the meat properly, it is safe to eat if it is within this 3 to 5 day window after purchase.
I have attaced a link to the USDA website that goes into detail about this information.
Feel free to post any follow-up questions you may have.