First of all you need to enable the checkbox “Enable the WordPress, Movable Type, MetaWeblog and Blogger XML-RPC publishing protocols.” in the menu “Settings”/”Writing”.

Now I searched on the web for a ready-to-use solution as I was probably not the first one who wanted to accomplish that. And as expected there were also many PHP scripts doing exactly that, but with one problem:
I wanted to set also the publishing date manually with the PHP request and most scripts I’ve found did not even try to do that at all.

Few scripts in fact did try to set the publishing date, but the result was an error message similar to:
Fatal error: Call to a member function getIso() on a non-object in /var/www/wp-includes/class-wp-xmlrpc-server.php on line 2373

After reading some documents regarding this strange ISO 8601 date format that is needed there, I found a solution myself:

/**
 * Performs a XML-RPC to a WordPress blog and adds a new post.
 *
 * @param string $title Title of the new post.
 * @param string $body Body text of the new post.
 * @param string $rpcurl RPC-URL of the blog (normally xmlrpc.php in the root directory).
 * @param string $username User name used to add the new post.
 * @param string $password Password used to add the new post.
 * @param string $categories Comma separated list of categories (make sure they already exist in your blog).
 * @param string $keywords Comma separated list of keywords/tags (in contrast to the categories they don't need to exist already).
 * @param string $pubdate Publishing date in ISO 8601 (with date('c') in PHP; current time is used if empty).
 * @param string $draftonly True to save as draft only, False to publish directly (default).
 * @return string The result of the XML-RPC.
 */
function wpPostXMLRPC($title, $body, $rpcurl, $username, $password, $categories, $keywords = '', $pubdate = '', $draftonly = false) {
	$content = array(
		'title' => $title,
		'description' => $body,
		'mt_allow_comments' => 1,
		'mt_allow_pings' => 1,
		'post_type' => 'post',
		'date_created_gmt' => '%pubdate%', // Just as place holder here.
		'mt_keywords' => $keywords,
		'categories' => array($categories)
	);
	$params = array(0, $username, $password, $content, !$draftonly);

	$request = xmlrpc_encode_request('metaWeblog.newPost', $params);
	// Now we need to set the real publishing date:
	$request = str_replace('<string>%pubdate%</string>',
			       '<dateTime.iso8601>' . $pubdate . '</dateTime.iso8601>',
			       $request);

	$ch = curl_init();
	curl_setopt($ch, CURLOPT_POSTFIELDS, $request);
	curl_setopt($ch, CURLOPT_URL, $rpcurl);
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
	curl_setopt($ch, CURLOPT_TIMEOUT, 1);
	$results = curl_exec($ch);
	curl_close($ch);
	return $results;
}

As you can see I inserted the data structure needed for the correct ISO 8601 date format myself directly into the XML request. This is probably not the most elegant way, but it works perfectly.

Did you come across the same issue already yourself?