5-field cron versus a seconds field

The five fields

From the left: minute (0–59), hour (0–23), day of month (1–31), month (1–12), day of week (0–6, with 0 usually Sunday). A star means every. 0 0 * * * is every day at 00:00. */15 * * * * is every 15 minutes. Ranges and lists work. Names for months and weekdays are a common extension, not something you should assume.

The day-of-month and day-of-week fields interact. In many implementations, if both are restricted, the job runs when either matches. That reads like “and” in English and behaves like “or.” Read the next run times. A sentence summary is not enough.

0 0 * * *

Where the sixth field comes from

Quartz and several CI systems put seconds first: second minute hour day month weekday. A line you copied from an AWS or Spring example will be shifted by one field if you paste it into classic cron. The minute becomes the hour, and the job runs at the wrong time or not at all.

DevTooKit’s parser and preview stay on five fields so the result matches crontab on a typical Linux host. If your scheduler documents a seconds field, generate the five-field body here and add the seconds field in that product’s UI. Do not expect this page to install the job. The host timezone is still the runner’s, not necessarily the laptop’s.

Read the next runs, then the shifted line

Parse 0 0 * * * and read the next five times. They should be midnights. Parse */15 9-17 * * 1-5 and confirm you see quarter hours inside office hours on weekdays, not a run at 09:07 and not a Saturday. If the English summary says “every day” while the list skips Saturday, believe the list. Day-of-week and day-of-month both set is the case to read twice.

Now take a Quartz line that starts with seconds, such as 0 0 0 * * *. In five-field cron that is “minute 0, hour 0, every day-of-month, every month, Sunday” only if you drop the first field correctly — and if you forget to drop it, the fields slide. The preview will show a schedule that is not midnight. That mismatch is the reason this tool refuses a sixth field instead of guessing which product you copied from.

Names, seconds, and year fields are extensions. A line that works in GitHub Actions may be rejected by Vixie cron, and the reverse. Generate the five-field expression, then paste it into the scheduler’s own validator before you depend on it. The preview uses a zone you can see. The server that runs the job uses its own zone, including DST. A 02:30 job on a spring-forward morning can be skipped or repeated. The expression did not change. The clock did.

Write the five fields in a comment above the line the first time you commit it. Future readers should not have to remember that the third star is day of month. The generator on DevTooKit is a way to produce that line from a sentence, and the preview is a way to check it. Neither one installs a crontab, and neither one knows whether your platform added a seconds field or a year field. If the docs show seven fields, you are in Quartz-like syntax and this parser will not describe that line honestly. Use it for the classic five, then adapt.

Related tools

• Cron Job PreviewPreview the next N runs of a cron expression with human-readable description and relative time.
• Crontab GeneratorVisually build a cron schedule from minute/hour/day/month/weekday pickers.