I have a client with whom I have a contract specifying payment in 30 days.

I have a client with whom I have a contract specifying payment in 30 days. (I'm on retainer). Every month, I have to go looking for payment. They always pay, but seem to have interpreted the contract to read, "after 30 days, this invoice can be treated like a timecard that gets put into the next payroll cycle". I look for payment around 10-12 days into the month so I know when it will hit the account, but shouldn't have to. My contract is up in a couple of months, and may or may not be renewed, depending on need. Do I request payment again this month (which still hasn't come on the 12th), stating the contract reads 30 days? Or do I let it slide because it does, eventually come? It won't break me to wait an extra couple of weeks, but it makes me feel a little less- than-taken -seriously. Then again, I don't want to give them pause to renew with me, as it's a good contract.