How do I convert a five-field cron expression to Quartz?

Add a seconds field at the start, put a question mark in whichever of day-of-month and day-of-week you are not restricting, and check the weekday numbers, because Quartz counts 1 as Sunday. For example 0 9 * * MON-FRI becomes 0 0 9 ? * MON-FRI. Expressions that restrict both day fields cannot be converted faithfully.

Does Kubernetes cron support seconds?

No. A CronJob schedule has five fields, so its shortest interval is one minute.

Why is my six-field expression rejected?

The scheduler probably expects a different layout. Some six-field dialects start with seconds, while AWS EventBridge starts with the minute and ends with a year. Check the documentation of the scheduler that will run the expression.

DevOps

Published October 11, 20267 min read

Five-Field vs. Six-Field Cron Expressions

The number of fields does not identify a cron dialect. Here is what changes between schedulers and why you cannot just add or drop a field.

People say 'cron expression' as if it were one standard, but the format has several dialects, and the number of fields is only the most visible difference. An expression that works in a crontab can be rejected, or worse, accepted with a different meaning, in Quartz, Spring or AWS EventBridge.

The layouts side by side

Dialect	Fields	Weekday numbering	Notable syntax
Unix/Linux cron	minute hour day-of-month month day-of-week	0-7, with 0 and 7 as Sunday	Macros such as @daily and @reboot
Kubernetes CronJob	same five fields	0-6, 0 is Sunday	Macros; timezone set in .spec.timeZone
Generic six-field	second minute hour day-of-month month day-of-week	Usually 0-7	Library-specific; confirm with yours
Quartz	second minute hour day-of-month month day-of-week [year]	1-7, 1 is Sunday	? L W # and an optional year
Spring	second minute hour day-of-month month day-of-week	0-7, 0 and 7 are Sunday	L, W, #, ? and macros
AWS EventBridge	minute hour day-of-month month day-of-week year	1-7, 1 is Sunday	cron(...) wrapper, ?, L, W, #; the year is required

Notice that six fields can mean 'seconds first' (Quartz, Spring) or 'minute first with a year last' (EventBridge). The same six tokens may therefore be valid in more than one dialect and mean different things.

Why the field count alone does not tell you the dialect

Take 0 0 12 ? * MON. The question mark suggests Quartz, but Spring also accepts it, and a generic seconds-based library might too. A tool can narrow the possibilities, but only you know which scheduler will run the expression. For that reason a good cron tool asks for the dialect and reports an ambiguity instead of guessing.

Never fix a field count by deleting a field

If a five-field scheduler rejects a six-field expression, dropping the first field is not a fix unless that field was 0 and the rest has the same meaning. Otherwise you silently change the schedule.

Converting between dialects

A faithful conversion adjusts three things: the field layout, the weekday numbers, and the day-field rules. Here is the same weekday schedule written for three schedulers.

0 9 * * MON-FRI           # Unix cron, Kubernetes (09:00 Monday to Friday)
0 0 9 ? * MON-FRI         # Quartz: seconds added, ? marks the unused day field
cron(0 9 ? * MON-FRI *)   # AWS EventBridge: year added, ? required

Using weekday names makes this much safer than numbers. Written as 1-5, the same schedule means Monday to Friday in Unix cron and Sunday to Thursday in Quartz.

Some expressions cannot be converted at all. Unix cron matches a day when either the day-of-month or the day-of-week matches, but Quartz and EventBridge allow only one of the two to be set. If an expression restricts both, there is no Quartz equivalent. The same applies in the other direction to operators such as L, W and #, which standard Unix cron lacks.

Checking a conversion

Two expressions that look alike are not necessarily equivalent. The Cron Builder on DevCraftKit checks an expression against each supported dialect, shows a conversion only when it can preserve the meaning, and compares the next run times of the original and the converted expression. A matching preview is good evidence over that window; it is not a mathematical proof.

Conclusion

Treat cron as a family (the basics are in Cron syntax explained). Decide which scheduler runs your expression, write it in that dialect, prefer names over numbers for weekdays and months, and verify the upcoming run times before relying on it.

Frequently asked questions

How do I convert a five-field cron expression to Quartz?: Add a seconds field at the start, put a question mark in whichever of day-of-month and day-of-week you are not restricting, and check the weekday numbers, because Quartz counts 1 as Sunday. For example 0 9 * * MON-FRI becomes 0 0 9 ? * MON-FRI. Expressions that restrict both day fields cannot be converted faithfully.
Does Kubernetes cron support seconds?: No. A CronJob schedule has five fields, so its shortest interval is one minute.
Why is my six-field expression rejected?: The scheduler probably expects a different layout. Some six-field dialects start with seconds, while AWS EventBridge starts with the minute and ends with a year. Check the documentation of the scheduler that will run the expression.

#cron #quartz #spring #kubernetes #eventbridge