From your questions:
1. If you submitting the whole form with a button, then:
Put this variable on the Onvisible of the screen:
Set(MyTimeNow,Now())
Then in the default of the datefield datacardvalue put:
If(YourFormName.Mode=FormMode.Edit,MyTimeNow,Parent.Default)
On hiding / disable the field, you can put this formula on the Displaymode of the field If(YourFormName.Mode=FormMode.Edit,View,Edit) . This will prevent them from manually changing the field.
2. But if you are to change on the datefield without submitting the whole form then on the OnSelect of the button put:
Patch('[dbo].[avlist] ',YourTexbox.Text, {DatefieldColumn:Now()})
