How to create custom “Duplicate” buttons for Adminhtml Edit Form and save custom module in Magento2.?

Create file to add button in VenderName/ModuleName/Block/Adminhtml/Edit/SaveAndDuplicateButton.php

Modify _construct function in Edit class to add button in

Add sub-query Magento 2 Collection

Magento 2 how to add new column in quote_item table?

You can add column like this

 

Add Date picker in phtml file Magento 2 Admin

For Datepicker :

For Date Time Picker :