I hope I got your question right and you can make use of this solution.
You can make this by creating a custom js file for the datepicker. I have managed to implement this by creating a file in my module under view/adminhtml/web/js/form/element called date.js (however, the location and the naming doesn't really matter as long as it is under the web folder).
Create the file and copy&paste this code:
define([
'Magento_Ui/js/form/element/date'
], function(Date) {
'use strict';
return Date.extend({
defaults: {
options: {
showsDate: true,
showsTime: true
},
}
});
});
When creating you form field, make sure to add the path to the file to the component node. That is, if you're using XML:
<item name="component" xsi:type="string">Vendor_ComponentName/js/form/element/date</item>
If you're building your form as in the catalog (meta arrays), do the following for your datetime eav attributes:
$meta['arguments']['data']['config']['component'] = 'Vendor_ComponentName/js/form/element/date';