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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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3 Answers

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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/libfaketime.so.1 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
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up vote 3 down vote

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
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up vote 3 down vote

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
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