@Override
  public String toString() {
    StringBuilder sb = new StringBuilder(512);
    sb.append(getDescription());
    sb.append(": status=");
    sb.append(getStatus());
    sb.append(", state=");
    sb.append(getState());
    sb.append(", startTime=");
    sb.append(getStartTime());
    sb.append(", completionTime=");
    sb.append(getCompletionTimestamp());
    return sb.toString();
  }
Force the completion timestamp backwards so that it expires now.
hbase-server/src/main/java/org/apache/hadoop/hbase/monitoring/MonitoredTaskImpl.java