Gridview Auto Update

I have a gridview where a user can select that a job is completed via a drop down and then select the date it was completed on, I've noticed that the date completed on field is not being updated sometimes.
I would like to fix this by basically running a piece of code which would set the Date_Completed to today's date when the user selects Completed, so essentially the date completed field would be updated automatically. I'm using vb.net.

Any ideas on how I would go about this ?