If you choose to bill subscribers upon sign up and bill subsequently on a specific calendar day you may use a buffer to prevent the first two billing cycles from being too close to each other.
To explain how this works let's look at a hypothetical monthly subscription plan. The plan is structured to bill subscribers upon sign up and then process all future billing cycles on the 1st of each month.
Customer A signs up on May 5th and is billed upon sign up on May 5th, and then again on June 1. Customer B signs up on May 30th and is billed upon sign up on May 30th and then again two days later on June 1. The business wants to keep things as is for Customer A, but wants Customer B's second billing date to be July 1 instead of June 1. The business wants to make it so that anyone that signs up within 10 days of the 1st of the month has their payment counted towards that upcoming billing cycle.
This option can be enabled using the Buffer setting. To enable this setting set "Include Buffer?" to "Yes" and enter the number of days that you want to use as a buffer.
Note that this feature is only available if Prorated Billing is disabled.