"Many eggs reach stores only a few days after the hen lays them. Egg cartons with the USDA grade shield on them must display the "pack date" (the day that the eggs were washed, graded, and placed in the carton). The number is a three-digit code that represents the consecutive day of the year (the "Julian Date") starting with January 1 as 001 and ending with December 31 as 365. When a "sell-by" date appears on a carton bearing the USDA grade shield, the code date may not exceed 45 days from the date of pack."
And although it would be possible to ramble on about the Julian calendar and the Gregorian version, it is not really necessary as this post is only for food related dates. The Julian date as stated by the USDA is simply a sequential number starting on January 1 as 001. So, January 20th would be 020. When we get to February, just add 31 and February 1 is 032. So what is the advantage to checking the Julian date? You can determine exactly when the eggs were packed! It is a lot more precise than the “sell by” date. But, you really don’t have to know the exact Julian date when shopping for eggs. Just look for the highest Julian date and compare with the sell by date.
Plants not under USDA inspection are governed by the state laws where the eggs are packed and/or sold. Most states require a pack date as described in this post. For more information about state egg laws, contact your state's Department of Agriculture. And, there probably is an app for the Julian Date…