According to the U.S. Dept. of Agriculture Food Safety & Inspection Service (USDA/FSIS), "By understanding the coding on the egg cartons, chefs and bakers can determine the freshness of the eggs.  Each carton of USDA graded eggs must show the date of packaging, the processing plant number, and may include an expiration date.  USDA assures that all labeling and claims made on the carton are truthful and accurate.  To determine freshness, a Julian date or pack-date calendar can be used.  This three-digit code indicates the date of packaging, starting with January 1 as 001 and ending with December 31 as 365.  These numbers represent the consecutive days of the year.  You can store fresh shell eggs in their cartons in the refrigerator for four to five weeks beyond this date."