@Override
  public String toString() {
    if (!readable()) {
      return "ERR";
    }

    return NanoTimer.nanoTimeToString(this.cumulate);
  }
Format the elapsed time to a human understandable string. Note: If timer is never started,
"ERR" will be returned.
hbase-server/src/test/java/org/apache/hadoop/hbase/io/hfile/NanoTimer.java