FWIW, I've had issues with scheduleFunction() working UNLESS I added some amount of time to it, never 0 seconds. Was intermittent enough that I just started adding it by habit. Not sure this is correct procedure though.
My take on it was, by the time DCS did "Other Things" and got back to your fxn call that time has come and gone and that call was 'purged'. Basically it must be some amount of time > timer.getTime(), even if only, say, 0.01 - I have routines that splice multiple calls to this fxn, one right after the other in essence, and I have to have _some_ amount of time here or they get 'skipped'.
This is going back to DCS 2.5, so try this:
timer.scheduleFunction(TRMS.DisplayCountdown, TRMS, timer.getTime() + 1)
