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From a server hosted in Poland (UTC +01:00), is there a way I can consistently have a crontab entry run at 9am New York time (UTC -05:00)?

For me this wasn't trivial since the daylight saving time ended last Sunday in Poland, so jobs that I have scheduled to run at 15:00 their local time most of the year are late this week by an hour from the point of view of US stock exchanges.

I remember Congress' decision a few years ago to extend DST, which put things out of sync for a few weeks per year.

One workaround I am not even sure would work (which I prefer to avoid anyhow) is that when I have a task:

0 14 * * * something

I fetch NY's time TZ=":US/Eastern" date +%s and compare using bash arithmetic whether I should sleep 3600 or not before running something

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Hmmm, you could pass the TZ environment variable to cron in its init.d script (TZ=':America/New_York') to have all your cron jobs run in New York time. BTW: New York is currently on DST, and is thus UTC-0400. – derobert Nov 1 '12 at 16:02
No; I don't want all, and according to some other posts, TZ doesn't necessarily affect crontab (in Ubuntu which I use it doesn't), unlike for date eg. TZ=":US/Eastern" date +%Y%m%d – Marcos Nov 1 '12 at 16:04
Minor update. en.wikipedia.org/wiki/Tz_database implies that since the DST rules for each zone are published into the TZ database, any up-to-date UNIX server should already have bundled knowledge of the wall clock time anywhere else in the world without having to consult over the network. – Marcos Nov 1 '12 at 17:30

2 Answers

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If its only one job, you can use a kluge like this:

# MM HH        DM MN DW   CMD
  0  *         *  *  *    TZ=':America/New_York'; if [ `date +\%H` -eq 9 ]; then actual-command-here; fi

I think you can set the environment variable like that in a crontab, if not just put the whole thing in a sh -c or a shell script...

I have a cron job that I need to run every other week which I use a similar kluge for.

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Thanks for the kluge; 9am was only a simplified example, in reality I have tasks sprinkled all over the clock and calendar. Developing on your idea, rather than stuffing my much longer, nastier crontab line into an if block, I could merely precede my commands with: ` [ $[$(date +%H) - $(TZ=":US/Eastern" date +%H)] == 5 ] && sleep 3600; w` where w is what I want to run at the right time – Marcos Nov 1 '12 at 16:19
sleep 3600 is only if you can only ever come in one hour early. What about when DST starts (Spring, instead of fall?) – derobert Nov 1 '12 at 17:33
The idea is that for most of the year, a 1-hour wait will not be in effect. Only during the few weeks of a 5-hour difference (whether spring or fall), rather than the typical 6, will sleep 3600 be executed. – Marcos Nov 1 '12 at 17:43
Um, the opposite of what I last wrote. – Marcos Nov 1 '12 at 21:53
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Okay, since altering the TZ env for date does seem to call up the correct current time in Ubuntu, this is at least a workaround (the one I was trying not to rely on):

SHELL=/bin/bash

0 14 * * 1-5  [ $[10#$(date +\%H) - 10#$(TZ=":US/Eastern" date +\%H)] == 5 ] || sleep 3600; w;df

So this aims to run w;df at 9am ET Mon-Fri. Instead of hour 15 I put in 14 with the possibility of sleeping one hour (3600 seconds) before running further commands.

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I don't like that 1. this isn't a host-timezone-indifferent solution, and 2. assumes the difference will never be greater than six hours, i.e., that DST in Poland will always end first and start last. – Marcos Nov 1 '12 at 17:52
In some cases got /bin/bash: 15 - 09: value too great for base (error token is "09") due to subtle problem of bash treatment of numbers beginning with 0 as octal. Fixed by preceding 10# to numbers. – Marcos Nov 5 '12 at 15:27

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