Advanced Date Picker - Waiting Day Ignored

Hi,

It could be a time zone issue. Try activating the "Set hour to noon when processing time" setting of the Hikashop Date Picker Plugin in the Joomla plugins manager.

Hi,

Then I'm not sure. Maybe the fact that it is enabled is the problem ?

The first thing to do would be to be able to reproduce the issue.
Since it is apparently hard to reproduce, it hints at a timezone issue: at some point of the day, the plugin thinks it is in the day before even though that's not the case in your country. And thus the waiting day is still enforced but it allows for today as it thinks it currently is yesterday.
So, I would recommend checking the time of the day that order was created on your website, and then try to reproduce the problem at that same time. Also, check the settings of the user account. On some Joomla websites, each user can configure his own timezone. That might mess up the current day calculations. Hopefully, you'll be able to reproduce the problem.
Once the problem can be reproduced consistently, we can look into it deeper.