There are three built-in date attributes for any record (document) in a database: addition date, creation date, and modification date. These are normally, respectively, the date the record was added to a database, the date the document was created, and the date it was modified (edited). Of these, users can change the last two: creation date and modification date. Since modification date can also be changed by editor programs and other events, it’s not safe to change it for a purpose such as the OP’s.
So, academics here have occasionally used creation date for the purpose of identifying historical event date. It is a manual process – you can can creation date for a single selected record in Get Info. Or you can use a script such as this one that was contributed by Frederiko. A forum search will reveal similar.
I have seen other researchers here and elsewhere use tags for the purpose successfully, since they are free-form and can be read externally (if the document is indexed and/or exported so that DEVONthink writes the tags to the file), or Spotlight Comment (for the same reason as tags). Another approach is to create a master document for each event you want to connect to other documents, and add links to the other documents into the master document so that it is a sort of index. Or, use the date in the name of the document – which can get cumbersome quickly.
Of course, yes, eventually there might be custom metadata. Anticipated for many years; not here yet.