When you add an extra and it asks how many days is required for it, I think think the expected delivery date for that extra is based on:
the date&time the original order was created+nr days delivery for original order+duration of any previous extras+duration of the new extra you just created.
And if you’ve done revisions if you enter “1 day” for the extra, the expected delivery date shown for the extra can be in the past. If you see the expected delivery date shown for the extra is wrong when you create it you could withdraw the extra and create a different one. So I think when you create the extra you need to enter the number of days based on the above, not from the current date/time.
Fiverr should make that extra thing (when an order is in progress) a lot clearer I think.