Tachometer for my bike

lastTime is the time in seconds at which the pin last changed state. When using edge:'rising' or edge:'falling', this is not the same as when the function was last called.

Yes, but it's easy enough to work around?

var lastTime;
var RPM;
var w=setWatch(function(e) {
  RPM = 60 / (e.time - lastTime);
  lastTime = e.time;
  counter++;
  },
  pin,
  {repeat: 'true', edge:'rising'}
);