Egg cartons from plants producing U.S Department of Agriculture (USDA)-graded eggs must display a Julian date – the date the eggs were packed. Although not required, egg cartons may also carry an expiration (sell-by) date and/or a best-by (use-by) date. On USDA grade-shielded egg cartons, if an expiration date appears, it can be no more than 30 days after the pack date. It may be less through the choice of the packer or quantity purchaser, such as your local supermarket chain. On USDA grade-shielded egg cartons, if a best-by (use-by) date appears, it can be no more than 45 days after the pack date. Eggs that are not packed under USDA's grading program must be labeled and coded in accordance with egg laws in the state where they are packed and/or sold. Most states require the use of a Julian date.
– See Julian Dates, Expiration Date