Which kind of date field are you using? I'm assuming you selected 'Date'. You need to be using a 'SaveDate' field:

Insert:Quick Parts:Field

And scroll down to select 'SaveDate' from the selection on the left. Then format as you need.


I think you're misreading the checkbox, it says 'Preserve formatting during updates.' This just means that the formatting (e.g. bold/italic/font/etc.) remains the same when the field updates.