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