def make_timestamp_range(start, end, start_timestamp_op=None,
    end_timestamp_op=None):
    """Create the query document to find timestamps within that range.

    This is done by given two possible datetimes and their operations.
    By default, using $gte for the lower bound and $lt for the upper bound.
    """
    ts_range = {}
    if start:
        if start_timestamp_op == 'gt':
            start_timestamp_op = '$gt'
        else:
            start_timestamp_op = '$gte'
        ts_range[start_timestamp_op] = start
    if end:
        if end_timestamp_op == 'le':
            end_timestamp_op = '$lte'
        else:
            end_timestamp_op = '$lt'
        ts_range[end_timestamp_op] = end
    return ts_range
