Client Script refuses to set an initial value in Subform field

Client Script refuses to set an initial value in Subform field

I tried a very simple, 1 line client script to set a default value in a custom subform field when the "Add Row" button is clicked and the user is entering data.

It does not work - can someone tell me why? ZDK documentation suggests this should be doable. Code:

ZDK.Page.getSubform('Attendance').setValues({ 'Workout_Date': new Date() });

It's being run on the "Edit - Standard" page. Instead of setting the default value in the empty row being added, it changes the OLD previously entered row data. Is there no way to set initial values?