In Salesforce Revenue Cloud, an Invoice Scheduler is used to automate the generation of invoices for billable order products. The Invoice Scheduler uses an Invoice Run to evaluate when and if an Order Product gets invoiced1.
When the Invoice Scheduler is set up with a Target Date (e.g., January 15) and Bill Usage Charges is set to False, it will generate invoice lines for Order Products with a Next Billing Date that is equal to or earlier than the Target Date1. This is because the Invoice Run is looking for eligible Order Products that match its criteria, which in this case would be Order Products with a Next Billing Date on or before the Target Date1.
It's important to note that the Invoice Scheduler kicks off Invoice Runs when it hits the start time that you specify. For example, if you have a monthly Invoice Scheduler that runs on the 15th of every month, it will generate invoices for all Order Products with a Next Billing Date of January 15 or earlier1.
Reference:
Use Invoice Scheduler to Generate Invoices - Salesforce
Using Invoice Schedulers to Automate Invoice Creation - Salesforce