Fix displaying calendar: reproduce same getAtDate method as Activity

src/Bundle/ChillCalendarBundle/Entity/Calendar.php

@@ -524,6 +524,16 @@ class Calendar implements TrackCreationInterface, TrackUpdateInterface, HasCente
         return $this->startDate;
     }
 
+    /**
+     * get the date of the calendar.
+     *
+     * Useful for showing the date of the calendar event, required by twig in some places.
+     */
+    public function getDate(): ?\DateTimeImmutable
+    {
+        return $this->getStartDate();
+    }
+
     public function getStatus(): ?string
     {
         return $this->status;