If it is still working, what's the big deal?
I have a bottle I got while buying other things. I used it for a couple rolls diluting some and it worked and I think the bottle was from the 1990s' if not 1980's. A lot of expiration dates are Cover You Against Liability sorts of things for companies. I've used Elon/Metol I bought in an antique shop (guesstimated bottling date 1940's). No, its not snowy white crystals but it makes just fine D-23. Admittedly, solutions are more of a problem than solids. Using Butane as a cover gas is interesting. I know from something I saw in Boy Scouts that narrowly avoided injuries that it is more dense than regular air and it is inert to any photo chemistry I can think of except using sparks to set off flash powder... I'd figured that if I was going to go the chemical preservation route that strenuously I'd get a bottle of Argon and regulator and some kind of hose at a welding supply shop and use that. Argon is extremely inert (won't burn, or do other chemistry in a photo situation) and heavier than air so ideal for purging chemical bottles of air/oxygen. Finally, if you are going to be that concerned about chemical longevity may I suggest using glass bottles, teflon or polyethylene lined caps and using electrical tape to seal the threads (two layers works best) and keeping the bottles in the fridge and allowing them to come to room temp before attempting to use. For serious, seriously air sensitive compounds it is best to take even more serious precautions, but nothing we do in a photolab degrades immediately or bursts into flame on contact with air.
Another thought: D-23 and Rodinal and Pyrocat HD all either keep really well as concentrates or are so inexpensive as to be used as one shot developers or both that it might make sense if you ever run out of the Tmax developer to consider them. D-23 can be mixed from 2 chemicals plus water. Mixing your own chemicals is a lot of fun and costs a lot less, typically.
