fix empty string timezone

include/datetime.php

@@ -80,6 +80,13 @@ function field_timezone($name='timezone', $label='', $current = 'America/Los_Ang
 if(! function_exists('datetime_convert')) {
 function datetime_convert($from = 'UTC', $to = 'UTC', $s = 'now', $fmt = "Y-m-d H:i:s") {
 
+	// Defaults to UTC if nothing is set, but not if set to empty string.
+
+	if($from == '')
+		$from = 'UTC';
+	if($to == '')
+		$to = 'UTC';
+
 	// Slight hackish adjustment so that 'zero' datetime actually returns what is intended
 	// otherwise we end up with -0001-11-30 ...
 	// add 32 days so that we at least get year 00, and then hack around the fact that