def _datetime_from_json(value, field):
    """Coerce 'value' to a datetime, if set or not nullable.

    Args:
        value (str): The timestamp.
        field (.SchemaField): The field corresponding to the value.

    Returns:
        Optional[datetime.datetime]: The parsed datetime object from
        ``value`` if the ``field`` is not null (otherwise it is
        :data:`None`).
    """
    if _not_null(value, field):
        if "." in value:
            # YYYY-MM-DDTHH:MM:SS.ffffff
            return datetime.datetime.strptime(value, _RFC3339_MICROS_NO_ZULU)
        else:
            # YYYY-MM-DDTHH:MM:SS
            return datetime.datetime.strptime(value, _RFC3339_NO_FRACTION)
    else:
        return None