diff --git a/db/upgrade.php b/db/upgrade.php
index 60d056f..1a42655 100644
--- a/db/upgrade.php
+++ b/db/upgrade.php
@@ -359,8 +359,9 @@ function xmldb_local_treestudyplan_upgrade($oldversion) {
                     // If no end time is given, assume a year duration for period calculations
                     $ydelta = (365*24*60*60)/$pcount;
                 }
-
-                $ydelta = $yend - $ystart;
+                else{
+                    $ydelta = $yend - $ystart;
+                }
                 $ptime = $ydelta / $pcount;
 
                 for($i=0; $i < $pcount; $i++){