How to fix YITH WOOCOMMERCE CHECKOUT MANAGER date conflict issue.

Add this code to the functions.php file of your theme, at the end of the file. You can also find the file in Appearance > Editor > Theme Functions (functions.php)

add_filter( 'woocommerce_billing_fields', 'bp_add_preorder_date_field_in_checkout',100 );

if( !function_exists( 'bp_add_preorder_date_field_in_checkout' ) ){
    function bp_add_preorder_date_field_in_checkout( $fields ){
        if ( 'no' === get_option( 'wc_preorders_always_choose_date' ) ) {
            $class = ['disabled-input', 'form-row-wide'];
        } else {
            $class = ['form-row-wide'];
        }
       
        $fields['preorder_date'] = [
            'label'             => __( 'Pre order Date', 'preorders-for-woocommerce-pro' ),
            'type'              => 'text',
            'class'             => $class,
            'description'       => __( 'Please enter the date when you want to receive your order', 'preorders-for-woocommerce-pro' ),
            // 'input_class'   => 'datepicker',
            'priority'          => 35,
            'required'          => true,
            'default'           => apply_filters( 'preorder_oldest_date', null ),
            'custom_attributes' => ['data-pre_order_date' => apply_filters( 'preorder_oldest_date', '' )],
        ];
        return $fields;
    }
}