The date is the REGISTRATION date. Some people were very organized and registered long before they actually left. Others were last-minute type people and registered around the same time they left. Others disappeared and were said to have left, but those would have been listed on a page for the "nonexistent" for a time.
Just make it clear the date is the registration date. I have no idea when any of these people emigrated. I might be old but I'm not 'that' old. ;-)