doc: Mention zeroed mtimes in the binary tarball.

Fixes <http://bugs.gnu.org/21111>.
Reported by Dave Love <fx@gnu.org>.

* doc/guix.texi (Binary Installation): Add paragraph about "implausibly old
  time stamps" warning from tar.

1 files changed, 6 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index a512f7dda91..09c7faf2546 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -308,6 +308,12 @@ This creates @file{/gnu/store} (@pxref{The Store}) and @file{/var/guix}.
 The latter contains a ready-to-use profile for @code{root} (see next
 step.)
 
+Some versions of GNU@tie{}tar raise a warning about ``implausibly old
+time stamps''.  This is because all the files in the archive have their
+modification time set to zero (which means January 1st, 1970.)  This is
+done on purpose to make sure the archive content is independent of its
+creation time, thus making it reproducible.
+
 Do @emph{not} unpack the tarball on a working Guix system since that
 would overwrite its own essential files.