code: def _utc(dt, source, field_name):
      
      if dt is None:
          raise dbt.exceptions.raise_database_error(
              "Expected a non-null value when querying field '{}' of table "
              " {} but received value 'null' instead".format(
                  field_name,
                  source))
  
      elif not hasattr(dt, 'tzinfo'):
          raise dbt.exceptions.raise_database_error(
              "Expected a timestamp value when querying field '{}' of table "
              "{} but received value of type '{}' instead".format(
                  field_name,
                  source,
                  type(dt).__name__))
  
      elif dt.tzinfo:
          return dt.astimezone(pytz.UTC)
      else:
          return dt.replace(tzinfo=pytz.UTC)
docs: If dt has a timezone, return a new datetime that's in UTC. Otherwise,
      assume the datetime is already for UTC and add the timezone.