If you only will be travelling in Switzerland you should consider buying a Swiss Travel Pass from the Swiss Railway. This pass includes more that the Swiss Pass from Eurail.

https://www.swiss-pass.ch/

I especially want to take the glacier express and the Bernina express. The latter also goes to Italy. Is it also possible to do that with the swiss travel pass? If so how long in advance may I buy this swiss travel pass? 

Yes, both of them are included in the same way as in the Eurail pass.  If you want to travel in the scenic panorama coaches you need a seat reservation, both if you have a Eurail pass and if you have a Swiss Travel Pass. 

Are you flying to Switzerland? In that case you can buy the Swiss Travel Pass at the SBB/CFF (Swiss Railway) ticket office at the airport. Or you can order it online and have it sent by mail. But then you have the risk that the mail gets lost. I personally prefer to buy it at the airport.

If you want seat reservations in the panorama coaches you can do that as soon as you know when you will be travelling.

Here is some information about scenic trains in Switzerland. 

https://www.swissactivities.com/en-ch/panoramic-trains-in-switzerland/

https://rail.cc/blog/interrail-route-glacier-express

Okay thanks for all that. But my main question is: how long in advance may I buy the ticket? Is it possible to buy it now but use it in July 2023? 

And btw: the swiss travel pass is a way more expensive than the interrail ticket. So please stop advertising your product. Thanks. 

It is not my product. I am a normal traveller who lives in Sweden and who has travelled a lot in Switzerland with the Swiss Pass. I only want to help. You need to compare the two passes and see what suits you the best. 

There is not ANY need to buy this pass so long advance. For RES it is not needed at all. PLus that a new flare of covid may also spoil all plans. You can even buy it on the spot for immediate use after you have landed at Züri airport. If you insist-use google (do people not even know anymore that exist?) and see the page for that.

Where as STP is a little more costly as IR/EUrail, it also offers a lot more-esp. for those touristy trains that the tipical tourist wants to use.

Passes can be activated up to 11 months after the date or purchase, so the earliest you can buy a Global pass for travel in July 2023 would be August 2022. Unfortunately we do not have a One Country pass for Switzerland. Indeed a more cost effective solution for you might be the Swiss Travel Pass as per Anna's suggestion, but unfortunately we cannot provide any support for this pass.