Hi there!
The wackiness you see there should've been fixed up in CheckBook 2.5.8 and later, when we made sure 4-digit years were used properly. We'll likely switch those date fields to date pickers, like in the Entry sheets, before too long.
You may not know there's an update because of an issue with the update checking code in version 2.5.7 and earlier. Visit
http://www.splasm.com/checkbook/update.html if you have CheckBook with the green pen in its icon or
http://www.splasm.com/checkbookpro/update.html if you have CheckBook Pro with the blue pen in its icon and grab the update to get things working the way you'd expect.