code: function hrtime(previousTimestamp){
    var clocktime = performanceNow.call(performance)*1e-3
    var seconds = Math.floor(clocktime)
    var nanoseconds = Math.floor((clocktime%1)*1e9)
    if (previousTimestamp) {
      seconds = seconds - previousTimestamp[0]
      nanoseconds = nanoseconds - previousTimestamp[1]
      if (nanoseconds<0) {
        seconds--
        nanoseconds += 1e9
      }
    }
    return [seconds,nanoseconds]
  }
docs: generate timestamp or delta see http://nodejs.org/api/process.html#process_process_hrtime