Can I change the bill date for all subscriptions to charge on the 1st or 15th of the month?

The billing date for the subscription is based on the activation date. This can be changed after the subscription is created either manually through the application UI or by utilizing the API. To change the renewal date moving forward you will want to visit the account and in the subscription summary window, click on "Options" then, "View Subscription" where you can change the billing date as needed.

You can also use the API to both set this date at the creation of the subscription and later on if you needed to change an existing subscription's renewal. As an example, if you're using the Create Subscription API call (https://dev.recurly.com/docs/create-subscription) then the first_renewal_date parameter should be used. This parameter specifies the subscription's next billing date even if it is different from the start/activation date. The first invoice billed will be prorated to ensure the customer only pays for the correct owed amount.