On a code of139C8, first 3 digits are Julian date. The C is the mfg plant. Next three digits is the Julian date. On second line, first digit is plant, middle 3 are Julian day of year. The next number is year, then 24 hour time.
Date is stamped.
Proof it by bringing water to the temperature in the instructions on the back. Stir in 1 tsp of sugar and one packet of yeast. After five minutes it should begin to bubble. At the end of 10 minutes, it should have a rounded crown of foam on it. If this happens, yeast is active.