Have you opened a jug of milk lately only to find it smells sour even though the sell-by date is still days away? Montana Families Report Milk Spoiling Before The Sell-By DateKTVQ reports that numerous families across Montana have been reaching out after finding their milk spoiled well before the date printed on the carton. By state law, that sell-by date has to be 12 days after the milk is pasteurized, so people are understandably wondering why it's going bad so early. Nick Northern//TSM-GF//Canva Nick Northern//TSM-GF//CanvaWhat Montana Families Should Do If Their Milk Spoils EarlyNow, that doesn't automatically mean the processing plant is responsible. If you've had milk spoil before the sell-by date, officials are asking you to check the processing code on the carton.