Hey @CurtBrink ,
You can first create a property in the document node and maintain it. To do it we can create a workflow with launcher will need a path of the folder where your Document will reside. In the backend, you can add a property of asset or doc with custom date modified property(which needs to maintain AEM/API).
So next time when you add a document in a folder then automatically you will get the property created with the date which will work for AEM.
Now For API also, whenever the call to API happens just pull the doc reference and adapt it and update the modified date property which we have created in the above step.
Let us know the rest.
Happy Coding 🙂
~BK