What happens if a Send goes through Journey Builder on a different date than the send date in DESelect Engage?

Sent through Journey Builder before the planned Send Date

DESelect Engage has reserved a timeslot for the campaign Send(also referred to as just "Send/s"), but for a later date than when you're actually sending it.

Hence, Engage will treat this like an unplanned Send, meaning it will check if there are available slots the moment the contact is hitting the Engage Journey Builder activity. To make this decision, it will look at recent past Sends, upcoming Sends (and their priorities), and the applicable rules, and determine if the unplanned Send can be sent today.

Sent through Journey Builder after the planned Send Date

DESelect Engage has reserved a timeslot for a Send for the planned Send Date.

If the Send is not being sent on the Planned Send Date, the timeslot is reserved for 3 more days.

If the Send is sent within 3 days of the Planned Send Date, DESelect Engage's outcome will still be Included for that Send, unless another Send comes into sight with a higher priority and there are not enough slots available to send both Sends. 
The same is true for retries, see this article for more info on that here.

If the Send is sent after those 3 extra days, DESelect Engage will consider this an unplanned Send and will check if there are available slots the moment the contact is hitting the DESelect Engage Journey Builder activity.