Clamp mtime to $SOURCE_DATE_EPOCH

[septatrix]

Discussed in #2571

^{Originally posted by septatrix March 30, 2024}

TL;DR: Current implementation of SourceDateEpoch= is very drastic as the mtime of every file is updated.

An alternative suggested in the discussion is to clamp the mtime of files newer than SourceDateEpoch= instead of setting it for all files. This is way less invasive and also the approach used by adopted by Fedora: https://fedoraproject.org/wiki/Changes/ReproducibleBuildsClampMtimes

[keszybz]

Yeah. In hindsight, this is a very specific implementation choice. Simple people want to just reset the times, but for most uses this is too much, because the timestamps of files installed from packages are correct and reproducible and we don't want to get rid of them.