<?php
add_filter( 'wp_insert_post_data', 'wpp_wc_save_post', '9999', 2 );

function wpp_wc_save_post( $data, $postarr ) {
	if ( $data['post_type'] == 'shop_order' )
		$_POST['order_date'] = gregdate( 'Y-m-d', $_POST['order_date'] );
	if ( $data['post_type'] == 'product' ) {
		if ( isset( $_POST['_sale_price_dates_from'] ) and ! empty( $_POST['_sale_price_dates_from'] ) ) {
			$_POST['_sale_price_dates_from'] = gregdate( 'Y-m-d', $_POST['_sale_price_dates_from'] );
		}
		if ( isset( $_POST['_sale_price_dates_to'] ) and ! empty( $_POST['_sale_price_dates_to'] ) ) {
			$_POST['_sale_price_dates_to'] = gregdate( 'Y-m-d', $_POST['_sale_price_dates_to'] );
		}
	}

	return $data;
}

function getGTLangCustomFieldValue( $metadata, $object_id, $meta_key, $single ) {
	if ( $meta_key == '_sale_price_dates_from' ) {
		var_dump( $_POST );
		var_dump( func_get_args() );//die;
	}
	if ( $meta_key == '_sale_price_dates_to' ) {
		var_dump( $_POST );
		var_dump( func_get_args() );//die;
	}
}

//add_filter( 'update_post_metadata', 'getGTLangCustomFieldValue', 10, 4 );