You can set up a date specific installment, such as payments to be made on the first of the month or on the date that a course starts.
You add your price as normal and then add an installment. Select "Wait for" and then "A date". You can then put in the date for the installment and an amount.
Note that if someone purchases after the installment date, they will be charged upon purchase the purchase amount plus any installments that have passed.