def get_file_date_part(now, hour) -> str:
    """ Construct the part of the filename that contains the model run date
    """
    if now.hour < hour:
        # if now (e.g. 10h00) is less than model run (e.g. 12), it means we have to look for yesterdays
        # model run.
        day = now.day - 1
    else:
        day = now.day
    date = '{year}{month:02d}{day:02d}'.format(
        year=now.year, month=now.month, day=day)
    return date