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I'm certainly trying to achieve something weird here, but I want to fake the date locally for a shell session on GNU/Linux. I need to black-box test how a program behaves at different dates, and modifying the system-wide date can have unwanted side effects (cron jobs, messed up logs, etc).

Any ideas ?

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4 Answers 4

up vote 11 down vote accepted

Haven't tried this one out yet. But if this is current is looks like someone already wrote the library you can preload with libfaketime.

The basic usage is:

user@host> LD_PRELOAD=/usr/local/lib/ FAKETIME="-15d" date
Mon Nov  8 12:01:12 CEST 2007

You can use ltrace to make sure all the time functions your application uses are covered.

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libfaketime is just perfect, thank you ! – Julien Nicoulaud May 4 '10 at 14:20

You might be able to preload a library that has an alternative time() implementation.

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Depending on how the program(s) access current time, you might have to preload alternative implementations of gettimeofday, clock_gettime, and/or possibly others as well, but yes, I've used this approach with success before. – Kjetil Joergensen May 4 '10 at 11:37
It's a Java program, and it seems to work well with libfaketime. – Julien Nicoulaud May 4 '10 at 14:29

You can set the TZ variable to an oddball value.

$ date
Tue May  4 06:24:43 CDT 2010
$ date -u
Tue May  4 11:24:47 UTC 2010
$ export TZ='CDT-3:12'
$ date
Tue May  4 14:36:53 CDT 2010
$ export TZ='CDT+5:37'
$ date
Tue May  4 05:48:00 CDT 2010
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Clever trick, but it only modifies the timezone, so this way you are limited to [-12 hours,+11 hours] range. – Julien Nicoulaud May 4 '10 at 14:04

You can just use executable faketime (from ubuntu repositories sudo apt-get install faketime) by:

faketime -f "-15d" date

Or even fake time in whole shell by

faketime -f "-15d" bash -l
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Kyle Brant posted pretty much this answer back in 2010. Please edit your answer to expand on it in some way that makes it substantially different from the existing answers. – Michael Kjörling Jan 9 at 13:06
My solution is much easier than using library preload as Kyle Brant answered because location of libfaketime .so file is very specific and depends on linux distribution and package managers. – abonec Jan 9 at 13:40

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