add accumulation of captured time

model/TimeSlot.py

@@ -42,3 +42,8 @@ class TimeSlot():
 
     def end_now(self):
         self.end = datetime.now()
+
+    def duration(self) -> datetime.timedelta:
+        end = self.end
+        if not end: end = datetime.now()
+        return self.end - self.start