The code on the yeast sachet is a Julian Date, representing the date that the yeast was packaged rather than a “Use By” or “Best Before” date. e.g. a sachet with 25007 was packaged on the 7th of September, which is the 250th day of 2007. While the beer kit, containing this yeast sachet, may have been produced on Thursday the 13th of September 2007, carrying a Best Before date of 13/09/09.