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Cron & RRULE Explorer

Explain schedules, calculate future runs, and compare cron with iCalendar recurrence.

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Two ways to describe recurrence

Cron & RRULE Parser Online — Explain Schedules and Preview Runs

Cron schedules system work by matching fields. RRULE expands calendar recurrence from DTSTART. They can produce similar dates, but their syntax and semantics are not interchangeable.

Cron expression · field-matching schedule
OR
iCalendar RRULE · calendar recurrence set

Decode the five POSIX fields

15 10 * * 1-5

At 10:15 on every weekday

15

minute

0–59

10

hour

0–23

*

day of month

1–31

*

month

1–12

1-5

day of week

0–7

Dialect boundary

POSIX / Vixie

5 fields

Seconds / Quartz

6–7 fields

Cloud schedulers

provider-specific restrictions

Expand recurrence parts from DTSTART

DTSTART;TZID=America/New_York:20260803T101500
RRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR;COUNT=8
DTSTART

Base date, time, and missing fields

FREQ

Required recurrence frequency

INTERVAL / BYDAY

Expansion and filtering

COUNT XOR UNTIL

Bound the recurrence

TZID

Interpret local wall time

Preview bounded occurrences in the selected IANA timezone

RUN 1

Mon 3 · 10:15

RUN 2

Wed 5 · 10:15

RUN 3

Fri 7 · 10:15

RUN 4

Mon 10 · 10:15

RUN 5

Wed 12 · 10:15

Spring gap

01:3002:00–02:59 skipped03:30

A local time can not exist. The scheduler may skip it or apply a policy.

Autumn fold

00:3001:00 occurs twice02:30

A local time occurs twice. Offset and scheduler policy determine which instant runs.

Deployment checklist

  • Choose and document the dialect
  • Store the intended timezone explicitly
  • Test near DST changes and month boundaries
  • Bound RRULE previews and untrusted input
  • Log future expected runs after deployment
  • Monitor missed or duplicate executions

Schedule parser questions

Are cron and RRULE interchangeable?
No. Cron matches time fields in a scheduler dialect; RRULE expands calendar recurrence from a start date with RFC 5545 rule parts.
Why does the same cron expression run at a different local hour?
The scheduler timezone or daylight-saving offset changed. Store the runtime timezone and inspect actual future instants, not only the English summary.
Can RRULE repeat forever?
Yes when neither COUNT nor UNTIL is present. The preview is deliberately bounded so untrusted rules cannot produce an unbounded computation.

Run a reproducible example

Use a fixed start instant for cron and a DTSTART-bound recurrence to make schedule examples repeatable.

Using an example replaces the current input, reference input, operation, and options. Public demonstration data only.

Preview five weekday cron runs

Unix cron 0 9 * * 1-5, UTC, starting just before 09:00 on Monday January 5, 2026.

Input

0 9 * * 1-5

The first run is that Monday at 09:00 UTC; weekends are skipped.

Next 5 cron runs in UTC
Monday, January 5, 2026

Expand a finite weekly recurrence

DTSTART fixes the first instant; COUNT=3 limits Mondays to three occurrences.

Input

DTSTART:20260105T090000Z
RRULE:FREQ=WEEKLY;COUNT=3;BYDAY=MO

January 5, 12, and 19 are listed in UTC. The requested preview limit does not override COUNT.

Next 3 occurrences in UTC
Monday, January 19, 2026

Continue with a related task

Unix Time Converter

Convert epochs and ISO dates with explicit units and timezone context.

Environment Matrix

Compare .env files, find missing keys, and create safe example files.

Text Diff

Compare two texts by line, word, or character in a local worker.

Repeat the task in DevSexy for Mac

Use the native app for repeated offline work and direct Option shortcuts.

Direct answer

What is Cron & RRULE Explorer?

Parse cron expressions or RFC 5545 RRULE schedules online, preview bounded future runs in a chosen timezone, and uncover DST or dialect traps before deployment.

What can it do?

  • Explain
  • Next runs
  • RRULE

How to use Cron & RRULE Explorer

  1. 01

    Prepare the input

    Identify cron versus RRULE and supply an explicit timezone and start instant.

  2. 02

    Choose the operation

    Preview runs instead of relying on field labels alone.

  3. 03

    Review the evidence

    Compare with your production scheduler, especially around daylight-saving changes and dialect-specific syntax.