Summary appears to take the date of the previous payment, and assume that the next one will be on the same day. My council tax usually goes out on the 25th, but due to the bank holiday, the August payment went out on the 27th. This caused Summary to assume that the next one would be the 27th, so it showed a positive balance. With no bank holidays in September, the payment went out as usual on the 25th, which caused a small cashflow issue as I happen to be paid on the 27th this month.
If we could manually override the date that Summary uses for these calculations for each DD, it wouldn’t need to assume the previous date.