SOURCE_DATE_EPOCH overwriting user specification

[kquick]

Describe the bug

In PR #12516 changes were made to replace the Copyright date specified via the copyright variable in conf.py with a SOURCE_DATE_EPOCH setting.

• This is not specified in the documentation anywhere, requiring code spelunking to determine why this is happening (esp. for build platforms where SOURCE_DATE_EPOCH is set to an unexpected value (this seems to be auto-set in some CI runner environments, yet not to a useful value).
• This replacement is done whether the user specified the copyright date manually or if they use the %Y substitution requesting the system provide the copyright. It seems wrong to override a user's explicit setting of the copyright date, and I would suggest that the SOURCE_DATE_EPOCH correction only be performed if the user specified the %Y substitution.

How to Reproduce

1. In conf.py, set copyright = '2024, me'
2. export SOURCE_DATE_EPOCH=1
3. run sphinx build and inspect the footer

Environment Information

Latest Sphinx, SOURCE_DATE_EPOCH=1

[AA-Turner]

Hi Kevin,

Please can you confirm that this changed in Sphinx 8.1? The first version of the SOURCE_DATE_EPOCH copyright substitution appears in Sphinx 1.4, from 2016 (7a89015).

I realise we don't document SOURCE_DATE_EPOCH, and this should improve. Would you be willing to suggest wording, or open a PR?

The '%Y' substitution is very new, I only added it in #12910 (first appearing in Sphinx 8.1). Perhaps we could change behaviour, but we must consider backward compatibility in any change.

A