I’m afraid I don’t have a clue as to what you mean by “validate the date”
If I recall correctly, the default theme has the date format for posts coded in, rather than respecting the format set in the admin panel.
If you want to change the date format you will have to edit the theme files. You will also need to learn how PHP controls the format of dates. This information is available from the PHP manual:
http://uk.php.net/manual/en/function.date.php
Come back here and ask if you get stuck.
Which theme file will I have to edit?
I figured it out. Had to make change in single.php, index.php and also archieve.php
