Anyone have a technique for getting the timestamp of when a new Lead entry page is opened and still retain the ability to use the standard Page Layout capability?

I know that this is possible if a custom VF page is constructed, but we want to use the standard Page Layout editor with Record Types, etc. 

There was a suggestion that the NOW() function could be put into a custom Lead formula field to capture when the page was opened then used in a before insert trigger to calculate the time difference, but the NOW() function seems to continuously update whenever the field is accessed.

Any suggestions would be greatly appreciated.  Thanks.