When a billing period is spread over two months, in many cases it would not be accurate or even possible to enter the billing data into one month. Here's how to do it accurately:
For a billing period with no seasonal transition, prorate the usage in accordance with the number of days in each month, then add it to that month. For example, a bill with 1000 kWh and billing period of June 20 to July 21 would have (11/32)*1000 = 343.75 kWh added to June and (21/32)*1000 = 656.25 kWh added to July. The rest of the usage for June would come from the bill prior to this billing period. The rest of the usage for July would come from the bill after. For a set of 12 bills, the partial usage for the last month recorded would be added to the partial usage of the first (and same) month recorded.
For a billing period with a seasonal transition (typically at the start of a month), the bill should automatically detail how much usage is in each season and therefore in each month. Billing periods with rate changes (that typically also occur at the start of a month) should also detail the usage for each month.
For a billing period with no seasonal or rate transition, enter the demand into the month with the most days. For example, a bill with 50 kW with a billing period of June 20 to July 21 would have 50 kW entered in for July. The demand for June would come from the prior billing period, which should have the majority of its days in June.
For a billing period with a seasonal or rate transition, the bill should automatically detail the demand for each month. To be completely accurate, compare the "minority" figure (the demand for the month with the fewer days) with the "majority" figure of the appropriate adjacent billing period and take the higher of the two. For example, a bill indicates a demand of 45 kW in September (in this case a summer month) but the billing period covers only 5 days in September. The prior billing period, which has the majority of its days in September, indicates a demand of 39 kW. In this case, enter in 45 kW, the higher of the two, for September.