add an explanatory comment to the Issue model preSave hack

src/IDF/Issue.php

@@ -161,6 +161,10 @@ class IDF_Issue extends Pluf_Model
         if ($this->id == '') {
             $this->creation_dtime = gmdate('Y-m-d H:i:s');
         }
+        // Note: If a due date is supplied then it must have
+        // a time appended to it so that calculations of when
+        // an issue falls overdue are accurate. In this case
+        // 23:59:59 is added to signify the end of the day.
         if ($this->due_dtime and !empty($this->due_dtime)) {
             $this->due_dtime = date('Y-m-d 23:59:59',
                 strtotime($this->due_dtime));