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From: ison <ison@airmail.cc>
Subject: Re: Version of package fontforge
Date: Wed, 5 Jun 2019 23:51:18 -0600
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To: Ricardo Wurmus <rekado@elephly.net>
Cc: help-guix@gnu.org

Assuming it would be ok to just set the date to the package version date, which
sounds reproducible to me, then using the date produced by:
date -u -d "20190317" +'%s'
gives: 1552780800

I did a quick test to set the configure-flags with this value:

  (arguments
    (append (package-arguments fontforge)
            '(#:configure-flags (list "SOURCE_DATE_EPOCH=1552780800"))))

And with that, if I run fontforge --version I get:

  Copyright (c) 2000-2018 by George Williams. See AUTHORS for Contributors.
   License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
   with many parts BSD <http://fontforge.org/license.html>. Please read LICENSE.
   Based on sources from 00:00 UTC 17-Mar-2019-ML-D.
   Based on source from git with hash: 
  fontforge 00:00 UTC 17-Mar-2019
  libfontforge 20190317

Which looks like the desired output.
However, the best solution would probably be to programmatically obtain
1552780800. I'm not sure what the best way would be to do that. I'm looking at
some of the available date commands inside SRFI-19 and I don't see anything for
converting a string like "20190317" to seconds.