So, I just stumbled across $ROB_TIMER in use -- wish I'd known about it before!
It's a free-running timer with a 1ms resolution. But, the System Variables manual leaves out one small, crucial detail: what its upper and lower limits are (and what the rollover behavior is).
Now, assuming it works the same as the $TIMER variables, just 12x faster, and that it's a "normal" 32bit signed integer, it should range from -2147483648 to +2147483647. This means that one full cycle should span a little over 49 days. So I don't have time to watch it roll over and see what it does.
I'd like to use $ROB_TIMER in place of some $TIMERs, but to bulletproof the function, I need to be sure of how it behaves at the edges.