[bee_rider]

If someone has overstayed you might want to note that down in case they come for another stay, right?

[arccy]

if you don't get any record of them leaving, from a manned border crossing or an airline reporting to you, then they've overstayed

[chatmasta]

You can also assume the first record of them entering another country is the date they left your country, since a person cannot be two places at once.