def get_prep_value(self, value):
        """
        Convert Python value to database value for QUERYING.
        We query with UTC, so this function converts datetime values to UTC.

        Calls to this function are followed by calls to ``get_db_prep_value()``,
        which is for backend-specific conversions.
        """
        log.debug("get_prep_value: {}, {}", value, type(value))
        if not value:
            return ''
            # For underlying (database) string types, e.g. VARCHAR, this
            # function must always return a string type.
            # https://docs.djangoproject.com/en/1.8/howto/custom-model-fields/
        # Convert to UTC
        return value.astimezone(timezone.utc)