Cron Expression Parser & Next-Run Visualizer

What is a cron expression?

A cron expression is a compact string that describes when a recurring task should run. It was invented for the Unix cron daemon in the 1970s and has become the lingua franca of scheduled jobs.

Each field maps to a time component. Classic Unix cron has 5 fields; modern Java frameworks (Spring, Quartz) add a seconds field at the start.

Field syntax

Position	Field	Range	Special
1	Second (Spring/Quartz only)	0–59
2 (or 1)	Minute	0–59
3 (or 2)	Hour	0–23
4 (or 3)	Day of month	1–31	L (last), W (weekday)
5 (or 4)	Month	1–12 or JAN–DEC
6 (or 5)	Day of week	0–6 or SUN–SAT	L, #

Within each field

• * — any value
• 5 — a specific value
• 5,10,15 — a list
• 5-10 — a range
• */15 — step (every 15 starting at 0)
• 5-20/2 — range with step
• MON-FRI — named values in day-of-week
• JAN, FEB — named values in month

Common patterns, decoded

Expression	Meaning
* * * * *	Every minute
*/5 * * * *	Every 5 minutes
0 * * * *	Every hour at minute 0
0 0 * * *	Every day at midnight
0 9 * * MON-FRI	Every weekday at 09:00
30 14 * * SUN	Every Sunday at 14:30
0 0 1 * *	Midnight on the 1st of every month
0 0 1 1 *	Midnight on January 1st (once a year)
0 12 15 * *	12:00 on the 15th of every month
0 */6 * * *	Every 6 hours (00:00, 06:00, 12:00, 18:00)

Scheduling with Spring @Scheduled

Spring uses a 6-field cron format (with seconds):

@Component
public class ReportJob {
    @Scheduled(cron = "0 0 9 * * MON-FRI", zone = "Europe/Paris")
    public void runDailyReport() {
        // Runs every weekday at 09:00:00 Paris time
    }
}

Enable scheduling on your application:

@SpringBootApplication
@EnableScheduling
public class App { public static void main(String[] a) { SpringApplication.run(App.class, a); } }

Best practice: always set zone explicitly. Without it, Spring uses the JVM default, which can drift between environments.

Scheduling with Quartz

Quartz is more powerful than @Scheduled: persistence, clustering, misfire handling.

import org.quartz.*;

JobDetail job = JobBuilder.newJob(ReportJob.class)
    .withIdentity("report", "group1")
    .build();

Trigger trigger = TriggerBuilder.newTrigger()
    .withIdentity("reportTrigger", "group1")
    .withSchedule(CronScheduleBuilder
        .cronSchedule("0 0 9 ? * MON-FRI")
        .inTimeZone(TimeZone.getTimeZone("Europe/Paris")))
    .build();

Scheduler scheduler = StdSchedulerFactory.getDefaultScheduler();
scheduler.start();
scheduler.scheduleJob(job, trigger);

Quartz uses ? for "no specific value" in day-of-month when day-of-week is set (and vice versa).

Scheduling without a framework

For simple periodic tasks without cron-level flexibility, ScheduledExecutorService is enough:

import java.util.concurrent.*;

ScheduledExecutorService sch = Executors.newSingleThreadScheduledExecutor();

// Every 5 minutes, starting now
sch.scheduleAtFixedRate(() -> runReport(), 0, 5, TimeUnit.MINUTES);

// Fixed delay between the end of one run and the start of the next
sch.scheduleWithFixedDelay(() -> runReport(), 0, 5, TimeUnit.MINUTES);

Use scheduleAtFixedRate for regular cadence, scheduleWithFixedDelay if a run's duration must not overlap the next.

cron-utils — advanced parsing

For programmatic cron parsing (validation, human descriptions, next-execution calculation) in Java, the cron-utils library is excellent:

<dependency>
  <groupId>com.cronutils</groupId>
  <artifactId>cron-utils</artifactId>
  <version>9.2.1</version>
</dependency>

import com.cronutils.model.CronType;
import com.cronutils.model.definition.*;
import com.cronutils.parser.CronParser;

CronParser parser = new CronParser(CronDefinitionBuilder.instanceDefinitionFor(CronType.SPRING));
Cron cron = parser.parse("0 0 9 * * MON-FRI");

ExecutionTime et = ExecutionTime.forCron(cron);
ZonedDateTime next = et.nextExecution(ZonedDateTime.now()).orElseThrow();

Common mistakes

• Confusing Unix cron and Spring/Quartz cron. Spring expects 6 fields, not 5. 0 9 * * MON-FRI in Spring means "at minute 9 on every hour on day-of-month MON to FRI" — not what you want.
• Forgetting DST. "Every day at 02:30" skips or repeats on DST transition days. Set zone = "UTC" if the exact moment matters more than the wall-clock time.
• Conflicting day-of-month and day-of-week. In Unix cron, specifying both broadens the match (OR). In Quartz, you must mark one as ?.
• Using 0 in the Sunday slot. Both 0 and 7 mean Sunday in most implementations — be explicit (SUN).
• Cron for sub-minute schedules. Unix cron can't run more often than once per minute. Use Spring/Quartz with seconds, or a ScheduledExecutorService.
• Assuming cron expressions validate in the IDE. Most IDEs don't syntax-check them. Always test with a tool like this one, or cron-utils.

Related tools and guides

• Epoch Timestamp — convert the next-execution timestamps
• DateTimeFormatter Playground — format output dates in your log
• JWT Decoder — tokens carrying their own expiration
• Java Online Compiler — try the Spring @Scheduled snippet