5204e5d0_dGrid_Documentation___SendGrid__Description
[Parameter] billing_date [Required] No [Requirements] If the new customer’s billing anniversary date is on a different date from the time you create the customer, you need to specify this parameter. Must be in YYYY-mm-dd format. This date must be within one billing cycle from the time you create this customer. (e.g. if they sign up on 2/28/11, then the latest date you can use is 3/28/11) [Description] 
If the new customer’s billing anniversary date is different from today (or when you execute this API call), you can specify the first billing anniversary date
Customer Management - SendGrid Documentation | SendGrid
Description