I also got this problem before. I try to use the (bool) but the problem still occured.
(bool) $instance['post_date']
will always return true unless $instance[‘post_date’] is an empty array, NULL or empty string
you can check this from http://php.net/manual/en/language.types.boolean.php
<?php
var_dump((bool) ""); // bool(false)
var_dump((bool) 1); // bool(true)
var_dump((bool) -2); // bool(true)
var_dump((bool) "foo"); // bool(true)
var_dump((bool) 2.3e5); // bool(true)
var_dump((bool) array(12)); // bool(true)
var_dump((bool) array()); // bool(false)
var_dump((bool) "false"); // bool(true)
?>
so to handle this problem, I print the $instance[‘post_date’] and get the value ‘on’. So now my code currently look something like this and working perfectly.
<?php checked( $instance['post_date'], 'on' ); ?>