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[GH-ISSUE #196] Problem with cron expressions #140

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opened 2026-02-25 23:41:19 +03:00 by kerem · 6 comments

kerem commented

2026-02-25 23:41:19 +03:00

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Originally created by @LordMike on GitHub (Oct 22, 2018).
Original GitHub issue: https://github.com/healthchecks/healthchecks/issues/196

Hi,

Very odd. The cron expression engine does not accept 0 2 * * *... It accepts 0 1 * * * and 0 3 * * *.

Originally created by @LordMike on GitHub (Oct 22, 2018). Original GitHub issue: https://github.com/healthchecks/healthchecks/issues/196 Hi, Very odd. The cron expression engine does not accept `0 2 * * *`... It accepts `0 1 * * *` and `0 3 * * *`. ![image](https://user-images.githubusercontent.com/1027111/47313789-1f871e80-d640-11e8-9b4c-4cb36e524494.png) ![image](https://user-images.githubusercontent.com/1027111/47313802-2746c300-d640-11e8-9dda-ca75e8cfcb7c.png) ![image](https://user-images.githubusercontent.com/1027111/47313827-3fb6dd80-d640-11e8-9500-03647f997f70.png)

kerem closed this issue

2026-02-25 23:41:19 +03:00

kerem commented

2026-02-25 23:41:20 +03:00

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@cuu508 commented on GitHub (Oct 22, 2018):

Thanks for reporting, looks like a bug indeed. It has something to do with time zones: if I select timezone "Europe/Riga", then "0 2 * * *" works but "0 3 * * *" does not.

Will look into this as soon as I can.

@cuu508 commented on GitHub (Oct 22, 2018): Thanks for reporting, looks like a bug indeed. It has something to do with time zones: if I select timezone "Europe/Riga", then "0 2 * * *" works but "0 3 * * *" does not. Will look into this as soon as I can.

kerem commented

2026-02-25 23:41:20 +03:00

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@LordMike commented on GitHub (Oct 22, 2018):

Indeed. Denmark/Copenhagen should be at +2 now, but we're changing in a few days (28th).

I imagine that's the issue. If I go for once a month, I get the correct offsets (we go to +1 there):

... but then this works, so maybe that was a red herring..

( I tested with Berlin, but Copenhagen is the same )

@LordMike commented on GitHub (Oct 22, 2018): Indeed. Denmark/Copenhagen should be at +2 now, but we're changing in a few days (28th). I imagine that's the issue. If I go for once a month, I get the correct offsets (we go to +1 there): ![image](https://user-images.githubusercontent.com/1027111/47314436-efd91600-d641-11e8-91ce-472fc2d4810f.png) ... but then this works, so maybe that was a red herring.. ![image](https://user-images.githubusercontent.com/1027111/47314455-02ebe600-d642-11e8-941e-e1ed733f2dc0.png) ( I tested with Berlin, but Copenhagen is the same )

kerem commented

2026-02-25 23:41:20 +03:00

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@cuu508 commented on GitHub (Oct 23, 2018):

This was indeed due to DST transitions. When clocks are rolled one hour back or forward, some local times happen twice and some are skipped over. Worked around the ambiguity but will need to do more investigation on how to best handle time jumps in March when DST starts.

This is a hairy problem. I would recommend to keep things simple and:

use UTC on your servers
if you're not using UTC, avoid scheduling cron jobs around 2-3AM local time ;-)

@cuu508 commented on GitHub (Oct 23, 2018): This was indeed due to DST transitions. When clocks are rolled one hour back or forward, some local times happen *twice* and some are skipped over. Worked around the ambiguity but will need to do more investigation on how to best handle time jumps in March when DST starts. This is a hairy problem. I would recommend to keep things simple and: * use UTC on your servers * if you're not using UTC, avoid scheduling cron jobs around 2-3AM local time ;-)

kerem commented

2026-02-25 23:41:20 +03:00

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@pkoziol commented on GitHub (Oct 29, 2018):

handle time jumps in March when DST starts

I believe European Commision (or Parliament?) voted/recommended recently to stop using DST, so this was probably the last time we changed our clocks in (most of?) the Europe.

@pkoziol commented on GitHub (Oct 29, 2018): > handle time jumps in March when DST starts I believe European Commision (or Parliament?) voted/recommended recently to stop using DST, so this was probably the last time we changed our clocks in (most of?) the Europe.

kerem commented

2026-02-25 23:41:20 +03:00

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@LordMike commented on GitHub (Oct 31, 2018):

They're not done yet. It's still in discussion, and the timetable differes between 2019 and 2021. We'll see :P

@LordMike commented on GitHub (Oct 31, 2018): They're not done yet. It's still in discussion, and the timetable differes between 2019 and 2021. We'll see :P

kerem commented

2026-02-25 23:41:20 +03:00

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@cuu508 commented on GitHub (Dec 14, 2018):

This project is using croniter (https://github.com/kiorky/croniter/) to parse and evaluate cron expressions.

Apparently croniter does work with timezone-aware datetimes too. Just updated Healthchecks to pass timezone-aware datetimes to croniter. By avoiding aware-naive-aware conversions we also avoid the ambiguities around DST transitions.

@cuu508 commented on GitHub (Dec 14, 2018): This project is using croniter (https://github.com/kiorky/croniter/) to parse and evaluate cron expressions. Apparently croniter does work with timezone-aware datetimes too. Just updated Healthchecks to pass timezone-aware datetimes to croniter. By avoiding aware-naive-aware conversions we also avoid the ambiguities around DST transitions.

kerem referenced this issue

2026-02-25 23:44:04 +03:00

[PR #140] [MERGED] Adding Content-Type header to Webhook integrations #899