Dior date codes began around 1997 and run on the same LVMH cipher as a 1990s Vuitton: two factory letters and four digits, with the first and third digits giving the MONTH and the second and fourth the year. The difference is that Dior never made Vuitton's 2007 switch to week numbers, so a 2015 Dior still encodes the month while a 2015 Vuitton encodes the week - the standard cross-house misreading. Modern codes add two leading digits that are a control number, not a date. This decoder applies Dior's rule, names the production country where the letters are documented, and checks the date against the selected model's history.
The code dates and places the bag; it authenticates nothing, and counterfeits carry correct-format codes. The leading two digits on modern codes are an unmapped control number - values above 12 are common, which is itself the proof they are not a month. Dior has never published the full factory-letter key: MA and BO are documented as Italy and MC as Spain, and for any other pair the made-in stamp resolves the country, which the code must then agree with. Pieces made before about 1997 carry no code at all, so absence on true early production - including the first Lady Diors - is normal. The date code is not the number on the paper card, which is the style number; and Dior issues no standalone authenticity card for its bags, so a listing selling the certificate is selling paper, not provenance.