Removing column from content type for folders only

Hello,  I've been searching and can't find a single thing about this.  I have a document library, the files in it have a required field of Start Date and End Date.

Because it's a required field, the folders also are required to have a start date and an end date.  The folders don't need start and end dates, only the files do.

I didn't think of this when the document library was set up, so I didn't care about the "Apply to all content types".  Now I want it removed from the folders only, but still apply to the files.  The issue is: There is no content type  for "Folder".

Well, there is actually, but I can't add it to the Content Type list.  I can add Document style content types, but not a folder.  Is there any possible way to add it to folders?  Or do I have to recreate the columns and reenter all of the data?

This document library doesn't have too many files, so I can do that - but I can see this being an issue later on.