What does "1% 10 Days" mean on a bill?

I got a heating bill that says "1% 10 Days" and "2% added after 30 days" on it. Presumably, if I pay the bill more than 30 days after it is due (which, ironically, is 10 days after they handed me the bill), then I will owe 2% more. But can I also ume that if I pay the bill in the next 10 days, I only need to pay 99% of the bill? I'd hate to presume incorrectly and piss off the oil delivery person.