Datepicker Incorrect date format

-- HikaShop version -- : 4.4.1
-- Joomla version -- : 3.9.25
-- PHP version -- : 7.4.16
-- Browser(s) name and version -- : Firefox dev. 87.0b9
-- Error-message(debug-mod must be tuned on) -- : Incorrect date format

No errors in joomla error console

Dear supportteam,
I have a problem with the datepicker.
When selecting the first available day the datepicker gives an error "Incorrect date format"
When selecting the day after the first available day all went fine.
Sundays an Mondays are checked as prohibited days and waiting days is set to 1

To be more precize you can't select a day next to a prohibited day.

Found something in Extensions-> Plugins -> Hikashop Date Picker Plugin there is one setting params time shift.
It looks like that solved the problem.