def date_adapter(obj, request):
    """Format the fields.Date to return String

    ::

        from pyramid.renderers import JSON
        from datetime import date
        json_renderer = JSON()
        json_renderer.add_adapter(date, date_adapter)
        config.add_renderer('json', json_renderer)


    :param obj: datetime obj
    :rtype: str, isoformat datetime
    """
    return obj.isoformat()
