Changing the date of the executable won't make the MacOS prefer it for opening documents.
In general (and there is more than a degree of black art behind this), the Mac OS (assuming you're using 9.x) uses the most-recently installed application to open a document. This is due to the way the Finder maintains the Desktop database of applications.
On this basis, you should be able to (re)install Acrobat Reader after installing Acrobat and the Mac OS should use that.
There are variations to this rule, however.
The Mac OS will use an application in the same directory as the document if one is found. This means that if your .pdf is in your Acrobat Reader folder, it will open with that.
I've also noticed, but haven't seen any hard documentation on this, that if you have an open Finder window with Acrobat Reader in it, it will get used in preference to Acrobat.
You might be able to use this to your advantage by placing Acrobat Reader on your desktop rather than buried in a folder. That way it should always be visible and be preferred over Acrobat. Even if this doesn't work, you have the ability to drag a .pdf and drop it in the Reader icon.
[This message has been edited by Camelot (edited 04-26-2001).]