def norm_dt(s):
    """normalize a datetime as string
    
    params:
        - s         str, as %Y-%m-%d %H:%M:%S, maybe without %H:%M:%S

    return: string %Y-%m-%d %H:%M:%S if input has %H:%M:%S
            otherwise appends 12:00:00 to %Y-%m-%d """
    ret = ''
    if len(s) >= 10 and len(s) < 19:
        ret = s[:10] + ' 12:00:00'
    elif len(s) == 19:
        ret = s[:]
    else:
        raise ValueError('{} is not acceptable as date/time'.format(s))
    return ret
