Debian and SOURCE_DATE_EPOCH=0

[Janneke Nieuwenhuizen]

Hi!

For quite some years now, I've been using in my makefiles somethhing
like

--8<---------------cut here---------------start------------->8---
# help2man reproducibility
SOURCE_DATE_EPOCH=$(shell git show HEAD --format=%ct --no-patch 2>/dev/null || echo 0)

# Be friendly to Debian; avoid using EPOCH
MTIME=$(SOURCE_DATE_EPOCH)

TAR_FLAGS=--sort=name --mtime=@$(MTIME) --owner=0 --group=0 --numeric-owner --mode=go=rX,u+rw,a-s
--8<---------------cut here---------------end--------------->8---

Now, I've somehow gotten entangled in the BLUE 0,1) build system
development and its lead devoloper, Olivier (cc) quite reasonably
challenged me: Why not just use --mtime=0 to make source tarballs
reproducible?

Using another value than 0 is non-trivial.  Easiest is to use the
timestamp of the latest commit in git (see the snippet above), but what
if there's no git available?

So my question is: Do we indeed advise not to use --mtime=0 for source
tarballs?  And if so, why is that?  And, does anyone remember where the
`Be friendly to Debian; avoid using EPOCH' comment may have originated?

And apart from the reprocudible source tarball, what about the value of
SOURCE_DATE_EPOCH we sue at build time, e.g., for tools like HELP2MAN.

Greetings,
Janneke

0) https://codeberg.org/lapislazuli/blue
1) https://fosdem.org/2026/schedule/event/3A7VGM-blue/

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Janneke Nieuwenhuizen <janneke at gnu.org>  | GNU LilyPond https://LilyPond.org
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