This is whats confusing. Invasions UI on the map tell you that there is a upcoming invasion and where.
But unless you can see a countdown timer on the top right of the “Sign Up At A Warboard” icon when you click on the settlement tab, then the invasion is going to be the next day instead of the date that is being displayed in the UI for the invasion.
I had to learn this the hard way because when I was low on Azoth, which I wasted to teleport to a settlement to catch an invasion that was listed, I noticed I couldnt sign up at the Warboard. Somebody had to explain the timer thing, which I never noticed before because you have to scroll down after clicking on the settlements.
Why do it this way though? Why not just display only current upcoming Invasions?