If I do not pay an electric bill on time, I will have an overdue bill. For example, the exceeding of the expected time is 2 months. So the payment is 2 months overdue. What do you call the period beyond the expected date of payment?
I think that for the specific case you are making they usually refer to a delay period which implies a delayed payment.
Prompt and delayed payment rates:
The prompt payment period extends up to the 21st day following and including the meter inspection day. When electricity bills are paid within such period, the prompt payment rate applies.
For payments made after the end of such period, a delayed payment rate that is 3% higher than the prompt payment rate applies. This is charged by adding the difference between the prompt and delayed payment rates for the present month to the following month's bill.
Other terms like delinquency and defaults periods have more serious financial and legal implications which you are not going to incur unless the payments are not made at all:
- The state of being late for a payment on a loan or other liability . If a payment is due on the third of each month and one does not pay until the ninth, the account is said to be in delinquency until the payment is made. Some liabilities have grace periods to allow for late periods up to a certain point without causing delinquency. A late fee may or may not be assessed on a delinquent payment. Serious and prolonged delinquency can lead to default.
(Farlex Financial Dictionary)