When you use on-demand rating and your rating group is by billing period, Zuora's rating engine determines the billing period in a different way as follows:

• From: the later of charge start date and current billing cycle day, with the day included.
• To: the earliest of billing target day, charge end date and the next billing cycle day, with the day excluded.

Take the following scenario as an example:

• Charge name: Charge 1
• Charge model: Tiered Pricing
• Trigger condition: Upon contract effective
• Billing period: Month
• Usage records rating option: On Demand.
• Price table:

Tier	Quantity From	Quantity To	List price
1	0	10	2.00
2	11	20	3.00
3	21	---	5.00

Step 1 : You upload the first batch of usages to Charge 1 as below:

Start Date	Quantity
01/01/2020	3
01/02/2020	5
01/03/2020	7

Step 2 : You create a bill run with a target date of 01/04/2020 and generate the first invoice.

In this case, the billing period is from 01/01/2020 (including this day) to 01/04/2020 (excluding this day), not the whole month of January .

Total quantity: 3 + 5 + 7 = 15

Usage cost on the first invoice: 10 * $2 + 5 * $3 = $35

Step 3 : You upload the second batch of usages to Charge 1 as below:

Start Date	Quantity
01/01/2020	1
01/04/2020	5

Step 4 : You create the second bill run with a target date of 01/05/2020 and generate the second invoice.

This time, the billing period is from 01/01/2020 (including this day) to 01/05/2020 (excluding this day) .

Total quantity: 3 + 5 + 7 + 1 + 5 = 21

Total usage fee: 10 * $2 + 10 * $3 + 1 * $5 = $55

The amount already billed is $35 as a result of the first bill run.

Usage cost on the second invoice: $55 - $35 = $20