def run_started_at(self) -> Optional[datetime.datetime]:
        """`run_started_at` outputs the timestamp that this run started, e.g.
        `2017-04-21 01:23:45.678`. The `run_started_at` variable is a Python
        `datetime` object. As of 0.9.1, the timezone of this variable defaults
        to UTC.

        > run_started_at_example.sql

            select
                '{{ run_started_at.strftime("%Y-%m-%d") }}' as date_day
            from ...


        To modify the timezone of this variable, use the the `pytz` module:

        > run_started_at_utc.sql

            {% set est = modules.pytz.timezone("America/New_York") %}
            select
                '{{ run_started_at.astimezone(est) }}' as run_started_est
            from ...
        """
        if tracking.active_user is not None:
            return tracking.active_user.run_started_at
        else:
            return None
`run_started_at` outputs the timestamp that this run started, e.g.
        `2017-04-21 01:23:45.678`. The `run_started_at` variable is a Python
        `datetime` object. As of 0.9.1, the timezone of this variable defaults
        to UTC.

        > run_started_at_example.sql

            select
                '{{ run_started_at.strftime("%Y-%m-%d") }}' as date_day
            from ...


        To modify the timezone of this variable, use the the `pytz` module:

        > run_started_at_utc.sql

            {% set est = modules.pytz.timezone("America/New_York") %}
            select
                '{{ run_started_at.astimezone(est) }}' as run_started_est
            from ...
        
core/dbt/context/base.py