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Cron expression parser and evaluator

Project description



Cron Sim(ulator), a cron expression parser and evaluator. Works with Python 3.8+. CronSim is written for and being used in Healthchecks (a cron job monitoring service).

Development priorities:

  • Correctness. CronSim tries to match Debian's cron as closely as possible, including its quirky behaviour during DST transitions.
  • Readability. Prefer simple over clever.
  • Minimalism. Don't implement features that Healthchecks will not use (for example, iteration in reverse, or the seconds field in cron expressions).


pip install cronsim


from datetime import datetime
from cronsim import CronSim

it = CronSim("0 0 * 2 MON#5",
for x in range(0, 10):


2044-02-29 00:00:00
2072-02-29 00:00:00
2112-02-29 00:00:00
2140-02-29 00:00:00
2168-02-29 00:00:00
2196-02-29 00:00:00
2208-02-29 00:00:00
2236-02-29 00:00:00
2264-02-29 00:00:00
2292-02-29 00:00:00

If CronSim receives an invalid cron expression, it raises cronsim.CronSimError:

from datetime import datetime
from cronsim import CronSim

CronSim("123 * * * *",


cronsim.cronsim.CronSimError: Bad minute

If CronSim cannot find a matching datetime in the next 50 years from the start date or from the previous match, it stops iteration by raising StopIteration:

from datetime import datetime
from cronsim import CronSim

# Every minute of 1st and 21st of month,
# if it is also the *last Monday* of the month:
it = CronSim("* * */20 * 1L",



CronSim Works With zoneinfo

CronSim starting from version 2.0 is designed to work with timezones provided by the zoneinfo module.

A previous version, CronSim 1.0, was designed for pytz and relied on its following non-standard features:

  • the non-standard is_dst flag in the localize() method
  • the pytz.AmbiguousTimeError and pytz.NonExistentTimeError exceptions
  • the normalize() method

Supported Cron Expression Features

CronSim aims to match Debian's cron implementation (which itself is based on Paul Vixie's cron, with modifications). If CronSim evaluates an expression differently from Debian's cron, that's a bug.

CronSim is open to adding support for non-standard syntax features, as long as they don't conflict or interfere with the standard syntax.

DST Transitions

CronSim handles Daylight Saving Time transitions the same as Debian's cron. Debian has special handling for jobs with a granularity greater than one hour:

Local time changes of less than three hours, such as  those  caused  by
the  start or end of Daylight Saving Time, are handled specially.  This
only applies to jobs that run at a specific time and jobs that are  run
with  a    granularity  greater  than  one hour.  Jobs that run more fre-
quently are scheduled normally.

If time has moved forward, those jobs that would have run in the inter-
val that has been skipped will be run immediately.  Conversely, if time
has moved backward, care is taken to avoid running jobs twice.

See test cases in, TestDstTransitions class for examples of this special handling.

Cron Expression Feature Matrix

Feature Debian Quartz croniter cronsim
Seconds in the 6th field No Yes Yes No
"L" as the day-of-month No Yes Yes Yes
"LW" as the day-of-month No Yes No Yes
"L" in the day-of-week field No Yes No Yes
Nth weekday of month No Yes Yes Yes

Seconds in the 6th field: an optional sixth field specifying seconds. Supports the same syntax features as the minutes field. Quartz Scheduler expects seconds in the first field, croniter expects seconds in the last field.

Quartz example: */15 * * * * * (every 15 seconds).

"L" as the day-of-month: support for the "L" special character in the day-of-month field. Interpreted as "the last day of the month".

Example: 0 0 L * * (at the midnight of the last day of every month).

"LW" as the day-of-month: support for the "LW" special value in the day-of-month field. Interpreted as "the last weekday (Mon-Fri) of the month".

Example: 0 0 LW * * (at the midnight of the last weekday of every month).

"L" in the day-of-week field: support for the "{weekday}L" syntax. For example, "5L" means "the last Friday of the month".

Example: 0 0 * * 6L (at the midnight of the last Saturday of every month).

Nth weekday of month: support for "{weekday}#{nth}" syntax. For example, "MON#1" means "first Monday of the month", "MON#2" means "second Monday of the month".

Example: 0 0 * * MON#1 (at midnight of the first monday of every month).

The explain() Method

Starting from version 2.4, the CronSim objects have an explain() method which generates a text description of the supplied cron expression.

from datetime import datetime
from cronsim import CronSim

expr = CronSim("*/5 9-17 * * *",


Every fifth minute from 09:00 through 17:59

The text descriptions are available in English only. The text descriptions use the 24-hour time format ("23:00" instead of "11:00 PM").

For examples of generated descriptions see tests/

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