diff -r eadaf0b8f02e -r a9b98b16b053 server/src/app/Libraries/Utils.php
--- a/server/src/app/Libraries/Utils.php	Fri Nov 27 17:59:36 2015 +0100
+++ b/server/src/app/Libraries/Utils.php	Tue Dec 01 13:33:08 2015 +0100
@@ -15,7 +15,7 @@
      */
     public static function dateIntervalToMillis(\DateInterval $di) {
         if(is_null($di)) {
-            return 0;
+            return null;
         }
         $seconds = ($di->s)
          + ($di->i * 60)
@@ -35,9 +35,13 @@
      * @return the number of milliseconds or 0 if $str is null
      */
     public static function iso8601IntervalToMillis($str) {
-        if(is_null($str) || $str === '') {
+        if(is_null($str)) {
+            return null;
+        }
+        elseif( $str === '') {
             return 0;
         }
+
         $di = new \DateInterval($str);
         return self::dateIntervalToMillis($di);
     }