When a Future Billing Date is set, the initial charge will be created on activation. The initial charge will be calculated for the period between the date of commencement of subscription (post activation) till the Billing date.

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