Hi,
What should I be looking for when checking the timezone setting of the joomla configuration and the timezone of the web server?
You have to check that the timezones are the same one on your Joomla configuration and on your server.
This can be causing your issue, if you select a day so with the timestamp based on that day at midnight and you don't have the same timezones so it can take into account the previous day.
That's why we asked to:
Activate setting "Set hour to 12:00 when processing time".
This allow some times to solve this issue, it avoid to have this difference for the hours changing the day.
I was not able to reproduce your issue of "incorrect date...", but this is maybe happening because it changing the day with that lag.
So a solution could be to try to edit the file "plugins/hikashop/datepickerfield/datepickerfield_class.php" and in the function "getTimestamp()" replace the line:
By another hour value to handle the lag.
ps: the option "Set hour to 12:00 when processing time" must be enabled.