When Calendar Billing is enabled with 'Immediate' alignment, the subscriptions will follow the billing date that is set and prorated charges will be collected.

We have migrated to new Self-Serve Calendar Billing configuration from 23 March 2023. Self-Serve Calendar Billing allows you to add multiple Site-level & Plan-level configurations also supports Yearly billing frequency. With this new configuration, you can manage both SaaS and E-Commerce business. Besides, the Advance trigger allows you to set calendar billing for specific events. Reach out to our support to enable this feature on your site.
Refer to this link on how to configure Self-Serve Calendar Billing for SaaS business
Refer to this link on how to configure Self-Serve Calendar Billing for E-Commerce business

Example: When the common billing date is set as 15th and a customer signs up for a $100 plan on the 2nd of a month, a prorated charge of ~$47 will be collected from the customer for the term 2nd to 15th.

However, if a past start_date is specified while creating the subscription, Calendar Billing will be applied with 'Delayed' alignment and not 'Immediate' alignment. 

Example: When the common billing date is set as 15th and a subscription is created ($100 plan) on Jan 2nd with the start_date specified as Jan 1st, the following will be the charges:

$100 collected on January 2nd for the period January 1st (according to the start_date) to February 1st

~$50 collected on February 1st for the period February 1st to February 15th