def format_date(start_date, format, offset=None):
    """Format a date given an optional offset.

    :param start_date:  an Arrow object representing the start date
    :param format:      the format string
    :param offset:      an offset measured in days
    :return:            a formatted datestring
    """

    if offset is not None:
        start_date = start_date.shift(days=offset)
    return start_date.format(format)