https://www.endpointdev.com/blog/2013/04/avoid-200-and-300-am-cron-jobs/

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Avoid 2:00 and 3:00 am cron jobs!

Jon Jensen

By Jon Jensen
April 8, 2013

A word to the wise: Do not set any cron jobs for 2:00 am or 3:00 am
on Sunday morning! Or to be safe, on other mornings besides Sunday as
well, since jobs originally set to run on some particular day may
eventually be changed to run on another day, or every day.

Most of the time such cron jobs will run fine, but if they run every
Sunday morning, then twice per year they will run at the exact time
daylight savings time (aka summer time) kicks in or ends, sometimes
with very strange results.

On Linux with vixie-cron we saw two cron jobs run something like once
per second between 3:00 and 3:01 when the most recent daylight
savings time began. Thus they ran about 60 times, stepping all over
each other and making a noisy mess in email. No serious harm was
done, but that's only because they were not tasks capable of causing
serious harm.

Feel free to wish for or agitate for or fund or write a better open
source job scheduler that everyone will use, one that will ensure no
overlapping runs, allow specifying time limits, etc. Better tools
exist, but until one of them achieves cron's level of ubiquity, we
have to live with cron at least some places and sometimes.

Alternatively, where possible set the server timezone to UTC so that
no daylight savings changes will happen at all.

Or most preferable: Governments of the world, stop the twice-yearly
dance of daylight saving time altogether.

But in the meantime this particular problem can be entirely avoided
by just not scheduling any cron jobs to run on Sunday morning at 2:00
or 3:00 server time.

devops linux sysadmin

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