On Sat, May 02, 2020 at 09:37:40AM +0200, Marcin Borkowski wrote:
> 
> On 2020-04-30, at 07:02, Kyle Meyer <kyle@kyleam.com> wrote:
> 
> > And note that a utility like datefudge or libfaketime is useful for
> > testing these sorts of things out.  For example:
> >
> >   $ datefudge "2020-02-18" emacs [...]
> 
> Shameless plug: I wrote about this use-case of datefudge sime time ago:
> http://mbork.pl/2019-08-05_datefudge_and_agenda_testing
> 
> (I don't know libfaketime).

It just plays games with LD_PRELOAD to trick the application (which is
supposed to use the usual libs when asking for time, but most do that).

Infinitely more lightweight than a container or a VM. On Debian:

  tomas@trotzki:~$ apt show libfaketime
  Package: libfaketime
  [...]
  Download-Size: 31.2 kB
  APT-Sources: http://ftp.de.debian.org/debian buster/main amd64 Packages
  Description: Report faked system time to programs (preload library)
   The Fake Time Preload Library (FTPL, a.k.a. libfaketime) intercepts
   various system calls which programs use to retrieve the current date
   and time [...] FTPL allows you to specify both absolute dates (e.g.,
   2004-01-01) and relative dates (e.g., 10 days ago).

You might need a VM for an app which bypasses the "usual libraries",
but then, I don't know whether I would like to have such a thing on
my box. Probably not without a good reason :-)

Cheers
-- t