If I try to change the start date and click on the “The payment I’m looking for isn’t there” button, I get presented with two different suggestions for adding money - Salary and Bank Transfer. So it’s not forcing to have salary added unless you’re seeing a completely different screen to me.
Anyway, it seems they’re also working on an option to define budget periods by specific dates if you don’t want to define by a specific incoming payment.
https://community.monzo.com/t/summary-feedback-thread/38572/700?u=osm