Calculated Field in Writeback table

Hi all,

I have a vizlib writeback table set to Update that looks like this

If I set the Audit Assign Date to the field "Audit Assign Date" and manually update the field, the table updates just fine. See below:

However, I want that field to be hidden and automatically set to today's date if someone updates other fields on a record. 

I set the field to =if(len("Audit Assigned Date")>0,"Audit Assigned Date",date(today())) and update a record, it creates duplicates in the table. What am I doing wrong here. Any ideas? I'd like the field to update similarly to if I was updating it manually.