You are too early for all of this.

No worries needed. In December, usually around the 2nd week, there’s a big international railway timetable change. This year the change is on 14th of December. Every trip on or after that date is not searchable yet as planning the new timetables is still an ongoing chore with every railway company at the moment.

New timetables and schedules will slowly start to become availble from October, in some countries only some weeks before the december date (Italy and Spain can be notoriously late on this).

If you want to get an indication of travel times, just plan a random day next week. Travel times and departures don’t change that much, usually a couple of minutes difference, if any difference already with the current one.

Try again in a couple of months.

Most reservations are available at the earliest three months before the date of travel.

Note that you can also take normal hourly regional trains from Chur to Tirano, without any reservation surcharge. Just hop on any regional train you want to Tirano. This is usually a much better experience than the (crowded and busy) Bernina express

In addition you can also board the regular carriages at the front of the Bernina Express: reservation-free, less busy and pull-down windows for a cool experience!

Thank you @BrendanDB for your detailed response.  Really helpful.  Is it then correct to say that there isn’t a single date and time when those days would be available to reserve?  

Thanks @thibcabe .  We are considering that option now.  Seems to offer more flexibility to the schedule also. 

Definitely not before October. These trains won’t sell out quickly anyway, they couldn’t even as seat reservations are not required!

(Obviously a good idea to make reservations around the busy Christmas period for peace of mind but a few days in advance would be enough.)

Thanks @thibcabe we’ll wait to reserve seats in October.