Hi,
Thank you for your feedback.
Thanks to that, and after numerous tests, I think we found the cause and the solution.
It happens only when the current date is in a different summer/winter time range than the date you're entering. So if you're in summer time and you enter a date in summer date, or if you're in winter time and enter a date in winter time, it works fine.
Change the line:
return $date - $timeoffset *60*60 + date('Z');
to:
return $date - $timeoffset *60*60 + date('Z', $date);
in the file administrator/components/com_hikashop/helpers/helper.php and that should fix the issue like it did on our end. We'll add that patch for the next version of HikaShop.