Funky milk?

It's the eternal question: How many days after the "Sell By" date stamped on a carton of milk is it still okay to drink the contents?

I pushed the limits today at breakfast. The date on the carton of milk was December 14. Today was December 17. Three days. The milk didn't smell funny but as I poured it over my Heritage O's and started eating, I thought I tasted something funky, something not quite right about those Heritage O's floating in a pool of white. I convinced myself that it was my imagination, that I was overly concerned for a date that read "Sell By" and not "Eat By," and that, even if it the milk was on the verge of funky, I could stomach it. I kept spooning those O's into my mouth, being careful not to scoop up extra milk.