How do I capture the modification date for a specific Column in SharePoint List?

My goal is to use a calculated field (for example "Backlog Start Date") that captures the modification date of a specific field in a SharePoint list (for example "Task Status") so that I can leverage the captured info in another calculated field to track the amount of time a record has been in a certain status (for example "Days in Backlog")

I'm referencing the default "Modified" column in my calculated field.  This is not ideal since any modification to the record (for example, adding a comment to the record)  resets the "Backlog Start Date" and skews the "Days in Backlog" data.  

Any recommendations? It would be great if there were a way to capture the modification timestamp of a specific field rather than the entire record, however it is unclear if this is an option in SharePoint?

Thanks