I'd suggest using an event handler to set up form 'dynamic' default values and/or configs.
I usually use the activate event of the form panel which is handy because it's fired every time the form is actually shown inside its container (the main Viewport, a navigation view, a tab panel or whatever). This way you can use the same form panel both for inserting and editing purposes, and reload the default value whenever there's no record - thus no value - set to the form.
Here's a quick example:
Code:
var field = this.getFields('field_name_config');
if (!field.getValue()) {
field.setValue(new Date());
}
Otherwise you can also rely on the initialize event of the field itself, but it will trigger only once upon the form panel's creation. In that case, you'll need to stick just this one-liner into the code editor:
Code:
this.setValue(new Date());