Compute next call of cron task using actual now

If the cron task took more time then the repeating delay, it is not useful to
reschedule it in the past.

trytond/trytond/ir/cron.py

@@ -123,7 +123,9 @@ class Cron(DeactivableMixin, ModelSQL, ModelView):
                 }),
             ]
 
-    def compute_next_call(self, now):
+    def compute_next_call(self, now=None):
+        if now is None:
+            now = datetime.datetime.now()
         return (now.replace(tzinfo=tz.UTC).astimezone(tz.SERVER)
             + relativedelta(**{self.interval_type: self.interval_number})
             + relativedelta(
@@ -210,7 +212,7 @@ class Cron(DeactivableMixin, ModelSQL, ModelView):
                         task.lock()
                     with processing(name):
                         task.run_once()
-                    task.next_call = task.compute_next_call()
+                    task.next_call = task.compute_next_call(now)
                     task.save()
                     logger.info("%s in %i ms", name, duration())
                 except Exception as e: