The goal of reproducible builds is that a rebuild of the same source
code with the same compiler, libraries, etc. should result in the same
binaries. SOURCE_DATE_EPOCH provides a standard way for build systems
to fill in the date of the latest source change, typically from a git
commit or from metadata like the debian/changelog in Debian packages.
This does not change anything if SOURCE_DATE_EPOCH is not defined;
the intention is that a larger build system like a Debian package
will define it.
Please see https://reproducible-builds.org/ for more information about
reproducible builds.
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