Fix value error

This pull request resolves the issue described in #55 (closed). :)

program/views.py

@@ -511,9 +511,9 @@ class APITimeSlotViewSet(viewsets.ModelViewSet):
         start = datetime.combine(date.today(), time(0, 0))
         end = start + timedelta(days=60)
 
-        if (start := self.request.query_params.get('start')) and (end := self.request.query_params.get('end')):
-            start = datetime.combine(datetime.strptime(start, '%Y-%m-%d').date(), time(0, 0))
-            end = datetime.combine(datetime.strptime(end, '%Y-%m-%d').date(), time(23, 59))
+        if ('start' in self.request.query_params) and ('end' in self.request.query_params):
+            start = datetime.combine(datetime.strptime(self.request.query_params.get('start'), '%Y-%m-%d').date(), time(0, 0))
+            end = datetime.combine(datetime.strptime(self.request.query_params.get('end'), '%Y-%m-%d').date(), time(23, 59))
 
         # Endpoints