@Override
  public String toString() {
    return "Schedule {" +
      "minutes='" + minutes + '\'' +
      ", hours='" + hours + '\'' +
      ", days_of_month='" + daysOfMonth + '\'' +
      ", month='" + month + '\'' +
      ", day_of_week='" + dayOfWeek + '\'' +
      ", year='" + year + '\'' +
      ", startTime='" + startTime + '\'' +
      ", endTime='" + endTime + '\'' +
      '}';
  }
Return a cron expression from the schedule fields.
Example: "0 0 12 * * ?"
@return
ambari-server/src/main/java/org/apache/ambari/server/state/scheduler/Schedule.java