I've never tried it through the app, I've always tried it on my computer. It was very clear on my computer about changing the date. Some have changed their shipping info and it never changed their date. For me every time I've tried to change something it wants to push my delivery date out one day. I think it just depends on your city, the Hub, ones that may have strange hours, etc.
But it was very clear on the computer each time I clicked to change, that it was pushing my date out. I don't know if the message with that is more clear or the desktop version than it is on the app.