Hi,
By default the time of the day selected in a custom field of the time advanced date picker is midnight.
So if you have a problem with the timezone configured in your Joomla configuration or PHP, etc it's possible that it shifts to the day before.
To counter than, we have a setting in the "hikashop advanced date picker field plugin" that you can enable so that it sets the hour as noon instead of midnight for the selected days. That way, even if there is an error of 1 or 2 hours with the timezones, you'll still get the correct dates.
You can find that setting by editing the plugin via the Joomla plugins manager.
Note also that it will only help for newly saved dates. It won't help for existing ones.