I think an automation is overkill for this. Here's how I would do it:
- Create a new field named Anticipated Purchase Date Compare (match the field type to whatever Anticipated Purchase Date is)
- Create a form rule that says: When Anticipated Purchase Date is not equal to the value in Anticipated Purchase Date Compare Action change Anticipated Purchase Date Changed By to the current user
- Create a form rule that says: When the record is saved and Anticipated Purchase Date Compare is not equal to the value in Anticipated Purchase Date Action change Anticipated Purchase Date Compare to the value in Anticipated Purchase Date
This will maintain a copy of the Anticipated Purchase Date from the last time the record was saved in a different field and whenever these two fields show different values, the first form rule will change the User field to denote who made the most recent change. The second form rule will then re-sync the two fields after the User field has been changed and the record saved. This prevents someone that is editing other fields in the record (but not touching Anticipated Purchase Date) from being noted as the person that changed the date fields. Hope this helps.
-Tom
