def get_expiration_time(self, app: Flask, session: SessionMixin) -> datetime | None:
        """A helper method that returns an expiration date for the session
        or ``None`` if the session is linked to the browser session.  The
        default implementation returns now + the permanent session
        lifetime configured on the application.
        """
        if session.permanent:
            return datetime.now(timezone.utc) + app.permanent_session_lifetime
        return None
A helper method that returns an expiration date for the session
        or ``None`` if the session is linked to the browser session.  The
        default implementation returns now + the permanent session
        lifetime configured on the application.
        
src/flask/sessions.py