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function custom_adjust_datepicker_range () {
	if ( is_checkout() ) {
		wp_enqueue_script( 'jquery' );
?>
<script type="text/javascript">
jQuery( document ).ready( function ( e ) {
	if ( jQuery( '.checkout-date-picker' ).length ) {
		jQuery( '.checkout-date-picker' ).datepicker( 'option', 'changeYear', true );
		jQuery( '.checkout-date-picker' ).datepicker( 'option', 'yearRange', '-40:+0' );
	}
});
</script>
<?php
	}
} // End custom_adjust_datepicker_range()
add_action( 'wp_footer', 'custom_adjust_datepicker_range', 50 );