That's not what I'm talking about.
In the main PHP file, you have this code:
if (!class_exists('hikashopNext7daysfield')) {
require_once __DIR__ . '/next7daysfield_class.php';
}
You've added it in the __construct method of the main class plgHikashopNext7daysfield
However, if you check the file plugins/hikashop/datepickerfield/datepickerfield.php you can see the corresponding code
if(defined('HIKASHOP_COMPONENT')) {
require_once( dirname(__FILE__).DS.'datepickerfield_class.php' );
}
outside of the class definition. I think it's probably why the system can't find the class definition, because the class file is not yet loaded.
Also, you're saying that you didn't make any change to the class file, but as far as I can see, the class file in your zip is quite different from what is in the default date picker field. When you're doing something like this, the best is to make a copy of the plugin, change only the filenames and classnames so that you can get the same result as the default one. And once that's working, you can go from there and start modifying things around for your needs. At least, that's how I do it.
For example, another issue you'll have is that you deleted the "show" method in your hikashopNext7daysfield class.
This method is used to display the entered value to the user. It is essential to have one, like in the default date picker field plugin.