Calendar invitees are not being updated with changes

I invited someone to an event. They accepted. Time changed, so I updated the change on my phone (original place from which the invitation was sent) and the update has not been recieved by the invitee. They have accepted the original invitation, but my phone says they have not.

Does me no good to invite someone when the entire system isn't functioning properly!

I'm going icloud to icloud account. Still not updating the invitee.

Help!