When scheduling a campaign for delivery in Section 3 (When) of the Campaign configuration page; the calendar will give you a few days in the past that you can select to schedule a Campaign.
Ex: Today is 3/29/20 but I am able to select 3/28/20 and 3/27/30.
Even with the option to available to select days in the past for a Campaign Start Date, the Campaign will not deliver when you try to schedule it and you'll receive the following error.
"Error creating schedule: Start Time Campaign schedule time should be in future" when creating a Campaign?
This is the current configuration of the platform and if you'd like to have it adjusted have your CSM submit Product Feedback. This request will go to the development to see if it can be added to the development roadmap.