Post-Dated Ponderances

I wrote a check for our water bill dated 12/16 – the day it’s due. I mailed the check earlier this week because I was at the post office anyway. It hit our account today, even though today is only the 10th. Are they allowed to do that? It’s not really that big of a deal. The money is there and if I were cutting it that close I wouldn’t have mailed it yet. But the check was postdated and the bank took it anyway? I guess I’m fuzzy on the rules about that. We’re almost completely online payments now, but there are a few checks still. Makes me wonder what other things can slip through…