WordPress at a glance
filter is not described

wp_checkdate filter-hook . WP 3.5.0

Filters whether the given date is valid for the Gregorian calendar.

Usage

add_filter( 'wp_checkdate', 'filter_function_name_1585', 10, 2 );
function filter_function_name_1585( $checkdate, $source_date ){
	// filter...

	return $checkdate;
}
$checkdate(true/false)
Whether the given date is valid.
$source_date(string)
Date to check.

Where the hook is called

wp_checkdate()
wp_checkdate
wp-includes/functions.php 5307
return apply_filters( 'wp_checkdate', checkdate( $month, $day, $year ), $source_date );

Where the hook is used (in WP core)

Does not used.