How the deadline behaves on the page
Set the reply date when you build the invitation and it lives in the guest's view, part of the same screen as the question, so nobody can honestly say they did not know. There is no separate terms email and no fine print; the deadline is written into the event the way the venue is.
As the date approaches, the guest list shows its blanks with precision, and one action sends a reminder to exactly those names on WhatsApp or email. Nothing fires on a schedule by itself: you choose the moment, the list chooses the recipients, and the message carries the page link so the reply can be finished on the spot. When the date passes, you finalise from the list you have, which is the version with no ghosts in it.