Have Agency "DateUpdated" field only change with agency manager action not site manager

It would be really great if the Agency DateUpdated field would correspond ONLY with the date that an agency manager actually logs in and updates that page. It is great that already the "login" date does not change when admin assumes the identity of an agency manager. It would make a huge difference to us if the agency update date could work the same way.

We are struggling with how to set some routine procedures to get agencies to login and actually look at the agency page to make updates and deactivate those that have not updated. We were using last login date, but its difficult to use when agencies have multiple managers. It takes multiple steps with the exported agency activity spreadsheet to find those that are truly delinquent. Lately we've been using the "update date" field as the required marker and email agencies if that date has not been updated in the last year. 

But that date field becomes unusable when system admin make changes to agency records. We are doing that by adding "tags" to indicate last date they attended an orientation or last date they submitted a required document. If we (admin) make these changes, or any changes within an agency record it makes that "update" date completely irrelevant. We'll go back to last login, but again, that is not easy to use.
Thanks 
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