hwclock does not accept date in local time

[Jacek Jaworski]

hwclock does not accept date in local time as documentation says.
man:
--date=date_string
[...]
The argument must be in local time, even if you keep your Hardware Clock in Coordinated Universal time. See the --utc option.

I have set locale to polish.
 $ locale -k LC_TIME
abday="nie;pon;wto;śro;czw;pią;sob"
day="niedziela;poniedziałek;wtorek;środa;czwartek;piątek;sobota"
abmon="sty;lut;mar;kwi;maj;cze;lip;sie;wrz;paź;lis;gru"
mon="styczeń;luty;marzec;kwiecień;maj;czerwiec;lipiec;sierpień;wrzesień;październik;listopad;grudzień"
am_pm=";"
d_t_fmt="%a, %-d %b %Y, %T"
d_fmt="%d.%m.%Y"
t_fmt="%T"
t_fmt_ampm=""
era=
era_year=""
era_d_fmt=""
alt_digits=
era_d_t_fmt=""
era_t_fmt=""
time-era-num-entries=0
time-era-entries="n"
week-ndays=7
week-1stday=19971130
week-1stweek=4
first_weekday=2
first_workday=2
cal_direction=1
timezone=""
date_fmt="%a, %-d %b %Y, %T %Z"
time-codeset="UTF-8"

Please note:
d_fmt="%d.%m.%Y"
This is polish local date format.

But:
$ sudo hwclock --localtime --set --date "06 07 2018 08:59"
date: błędna data: „06 07 2018 08:59”
hwclock: The date command issued by hwclock returned unexpected results.
The command was:
  date --date="06 07 2018 08:59" +seconds-into-epoch=%s
The response was:

hwclock: No usable set-to time. Cannot set clock.

"błędna data" means "wrong date" or "wrong date format"