The partial payment option was implemented because we wanted to help merchants like you to provide your customers with an easier way to manage their budgets more effectively while waiting for the stock of the product to be available.
That being said, the date of the 2nd charge will be based on the date of availability of the preorder product. As of the moment, you can't set an interval between the 1st and 2nd/last charge.
This also explains why the calendar lets you pick a date so that the system will automatically change the credit card of the customer which makes the process effortless.