In Magento 2 you can add Datepicker for a custom option Date Field in Product detail page using enable Javascript Calander. Native Magento doesn’t display Date picker for Date Custom Option field. They just show as a Dropdown option for Day, Month and Year.
You have to enable Javascript Calander from System Configuration.
Go To
Admin panel, Stores -> Configuration -> Catalog
Go to Section,
Date & Time Custom Options
Enable Use JavaScript Calendar to Yes,
Set Year Range from and to a field.
Save Configuration.
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You can check the Product with Date Custom option as Date picker instead of Default Drop down selection for Date Option.