I had something similar with my council last year. It was actually overdue. I told them (didn't ask) that I would pay them by standing order on the the 1st of each month, as I often don't get paid myself (I work for the same council) until the 30th or 31st , so 28th would be useless.
By the time they wrote back to say I couldn't do that, and that I should make two months' payment in the same month to even it out, I'd already made four payments under the proposal, with only another three payments to go. I pointed out that even if they wanted the account paid off by 28th January, they wouldn't get it until 1st Feb in any case. They haven't responded yet and now the account is paid.
I intend to pay by the same arrangement this coming year (only on time this time). I say when my money comes out of my account, not them.