Thank you Dan
My issue is that that function, "onTimeRangeRightClick", fires on the mousedown event, not on the mouseup.
So what would happen is:
I right click on a date, the event fires, I select a range and then release.
Instead of:
I right click on a date, I select a range, release and then the event fires.
My hacky solution is to use "onTimeRangeSelected", which fires after mouseup, to detect if the last button press was "left" or "right".