If the Start date and the billing date are set with the same date confusion occurs while creating a contract.
Here is a scenario were the Start date and the billing date are set as '1st' and it's set to quarterly billing.
Logically the bill should generate on the 1st of the fourth month as per the date entry in the contract, but if the bill generates on 1st of the fourth month the generated bill will be for 3 months and 1 day.
If the Billing Date was marked as 31st, then the bill would have been calculated till the 31st (or the last day of the month) of the third month and the bill would have been generated on 1st of fourth month. (Quarterly calculation - three month bill)
If this error has occurred then you can edit the existing contract and change the billing date as 31st to avoid this confusion in billing By setting this the bill gets calculated till last date of the third month and bill gets generated on the 1st of the fourth month.
Make sure the billing date is set as 31st because even if there are 30days in a month when 31st is set, the last day of the month will be calculated.
Else the bill will get calculated till the 1st of every month which might not seem to be periodical.