@Override
  public int compareTo(LruCachedBlock that) {
    // Newer accessed blocks sort before older ones.
    if (this.accessTime == that.accessTime) return 0;
    return this.accessTime < that.accessTime ? 1 : -1;
  }
Returns Time we were cached at in nano seconds.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/LruCachedBlock.java