This seems to be related to formslib, see detailed discussion at here:
http://moodle.org/mod/forum/discuss.php?d=69707

Basically if you enter the dates wrong, Moodle 1.8+ will just take it and convert it into some other dates without give the user any warning. Tim suggested that maybe some sort of javascript date selector widget from YUI (http://developer.yahoo.com/yui/calendar/) probably could fix it but not sure whether the codes would be really accessible. If someone could help fixing this that would be great, thanks!