Lago (YC S21 = Open-source metering and usage-based billing) founder here.

Billing cycles are often a source of engineering complexity and confusion, so we made billing periods flexible in Lago:

- If you want to migrate existing customers to Lago, you can set a billing period in the past. - If you just signed a new contract that will start in two weeks, you can set a start date in the future

Other elements of flexibility: - Switch from 'calendar' to 'anniversary billing periods' within Lago - Assign multiple plans to a customer, and meter and charge usage separately (e.g., if your pricing looks like Heroku, Shopify or Webflow, for instance)

Documentation: https://doc.getlago.com/docs/guide/plans/subscription#subscription-date

Github: https://github.com/getlago/lago