CRON expressions

import java.text.ParseException;
import java.util.Date;
import org.quartz.CronExpression;

public class CronTester {
 public static void main(String[] args) throws ParseException {
  final String expression = "0 0/30 01-06 * * ?";
  final CronExpression cronExpression = new CronExpression(expression);
  Date date = new Date();
  for (int i = 0; i < 20; i++) {
   date = cronExpression.getNextValidTimeAfter(date);
   System.out.println(date);
  }
 }
}

http://en.wikipedia.org/wiki/Cron
http://www.cronmaker.com/