Hello,
The point is that I've checked the Advance Date picker code, and I wasn't able to get the root issue, and I was only able to make test on your website.
From this it remains only one point, your server timezone :
Today I checked your Joomla Timezone settings, everything is okey on this side, but it remains something your server timezone.
When your HikaShop or your Joomla ask for have the time if your server timezone is different that can leads to this kind of situation.
You have to ask to your server support which timezone is used (if you haven't) and from there, you have to set 14 extra hour AND addition or subtraction the difference to your server timezone.
Concrete case :
Your local timezone is 14 hour but your server Timezone is in advance of 2 hour, you have to enter in your extra hour options 16 (hour).
Can you try this ? Hope this will help you.
Regards