How to use the taskcluster.utils.stringDate function in taskcluster

# Build complete namespaces list, with monitoring update
        full_namespaces = [
            TASKCLUSTER_NAMESPACE.format(channel=settings.app_channel, name=name)
            for name in namespaces
        ]

        # Index for all required namespaces
        for namespace in full_namespaces:
            self.index_service.insertTask(
                namespace,
                {
                    "taskId": settings.taskcluster.task_id,
                    "rank": 0,
                    "data": payload,
                    "expires": stringDate(now + timedelta(days=TASKCLUSTER_INDEX_TTL)),
                },

"""
        Index current task on Taskcluster index
        """
        assert isinstance(revision, Revision)

        if settings.taskcluster.local or self.index_service is None:
            logger.info("Skipping taskcluster indexing", rev=str(revision), **kwargs)
            return

        # Build payload
        payload = revision.as_dict()
        payload.update(kwargs)

        # Always add the indexing
        now = datetime.utcnow()
        payload["indexed"] = stringDate(now)

        # Always add the source and try config
        payload["source"] = "try"
        payload["try_task_id"] = settings.try_task_id
        payload["try_group_id"] = settings.try_group_id

        # Always add the repository we are working on
        # This is mainly used by the frontend to list & filter diffs
        payload["repository"] = revision.target_repository

        # Add restartable flag for monitoring
        payload["monitoring_restart"] = payload["state"] == "error" and payload.get(
            "error_code"
        ) in ("watchdog", "mercurial")

        # Add a sub namespace with the task id to be able to list

"""
        Index current task on Taskcluster index
        """
        assert isinstance(revision, Revision)

        if settings.taskcluster.local or self.index_service is None:
            logger.info("Skipping taskcluster indexing", rev=str(revision), **kwargs)
            return

        # Build payload
        payload = revision.as_dict()
        payload.update(kwargs)

        # Always add the indexing
        now = datetime.utcnow()
        payload["indexed"] = stringDate(now)

        # Always add the source and try config
        payload["source"] = "try"
        payload["try_task_id"] = settings.try_task_id
        payload["try_group_id"] = settings.try_group_id

        # Always add the repository we are working on
        # This is mainly used by the frontend to list & filter diffs
        payload["repository"] = revision.target_repository

        # Add restartable flag for monitoring
        payload["monitoring_restart"] = payload["state"] == "error" and payload.get(
            "error_code"
        ) in ("watchdog", "mercurial")

        # Add a sub namespace with the task id to be able to list

# Build complete namespaces list, with monitoring update
        full_namespaces = [
            TASKCLUSTER_NAMESPACE.format(channel=settings.app_channel, name=name)
            for name in namespaces
        ]

        # Index for all required namespaces
        for namespace in full_namespaces:
            self.index_service.insertTask(
                namespace,
                {
                    "taskId": settings.taskcluster.task_id,
                    "rank": 0,
                    "data": payload,
                    "expires": stringDate(now + timedelta(days=TASKCLUSTER_INDEX_TTL)),
                },