I'm not sure about the built in WM 6.1 threaded sms, but I know the Palm threaded SMS shows both the date and time of each message, regardless of how old it is. You can find it here:
http://discussion.treocentral.com/tr...-treo-pro.html
Use the first one (For Sprint Treo Pro). I'm using it on my Mogul with the Titan Reloaded 1.34 ROM and it works great, except that it doesn't display the number of unread text messages on the today screen (a bug with that version, the non-800w version worked fine). It will also allow you to send pictures without having to use the Sprint Picturemail program. Received MMSs will still comes as links though.
Before you try the Palm threaded SMS, see if the default WM SMS doesn't let you open each individual message. I know with the Palm one, you can highlight a message, then press enter or tap on it and view the individual message, where it should show the date and time. I haven't used the default one in a while though, so I'm not sure if that works.
Sorry if I typed a long novel lol, but I hope this helps.