In a date-based campaign: a campaign event is sent to all recipients on the same date. If a recipient is added to a date-based campaign after the campaign has begun, past events will not be sent. The new recipient will receive only events scheduled for future dates.
In a time-based campaign: a campaign event is sent to each recipient on a date relative to the date the recipient was added to the campaign. For example, if a campaign event is set to be sent 1 day after the recipient is added to the campaign, a recipient you add on the 1st of the month will receive that event on the 2nd of the month. If you add another recipient on the 5th of the month, that recipient will receive that same event on the 6th of the month.