From what I've been able to figure out, United is using block time now as opposed to mileage. However the "average block time for each city pair" that is used for meal planning is not published. For example: If SAT-DEN is blocked at 2:24 in winter going north in a CR7, but 2:06 in summer going south in a E175, then the average block for that city pair is now 2:15. That's the number UA uses for every flight connecting those two cities in either direction. And keep in mind there are countless aircraft, seasonal, and time-of-day differences that are all averaged together. So basically they ARE using block time, but there's no good way to know what block time they use.
Why they changed from the actual mileage I'll never understand.