add list of task

Entity/SingleTask.php

@@ -142,6 +142,28 @@ class SingleTask extends AbstractTask
         return $this->warningInterval;
     }
     
+    /**
+     * Get the Warning date, computed from the difference between the 
+     * end date and the warning interval
+     * 
+     * Return null if warningDate or endDate is null
+     * 
+     * @return \DateTimeImmutable
+     */
+    public function getWarningDate()
+    {
+        if ($this->getWarningInterval() === null) {
+            return null;
+        }
+        
+        if ($this->getEndDate() === null) {
+            return null;
+        }
+        
+        return \DateTimeImmutable::createFromMutable($this->getEndDate())
+            ->sub($this->getWarningInterval());
+    }
+    
     function getRecurringTask(): RecurringTask
     {
         return $this->recurringTask;