Invites can have a before buffer, after buffer, or both. When a buffer is added to an invite, it affects new events created from that invite as well as all previous events created from that invite. When an invite has a buffer time set, zcal won't allow an invite time slot to be booked if its buffer overlaps with any other event on your calendar. Likewise, for invites without buffer times, zcal won't allow a time slot to be booked if it overlaps with the buffer of an existing zcal event on your calendar.