I need to add a custom field in the publish entry area (specifically in the date tab). Should I use the Control Panel class or is there a preferred method?
If I understood correctly, you need to add an extra field to an already existing channel and you can easily do that from the CP by going to Admin > Channel Administration > Channel Fields.
Browse and click on the channel that you want to add your custom field to and create a new channel field.
To put your field under the Date tab: