Promote the default reproducible build date

[Sergey Ponomarev]

ok, sorry, I saw that many people from the repo builds are
intersecting with Maven people in discussions so I expected that
everyone in context.
Maven, following the Gradle now use the static date 1970-01-01 for
build timestamp
https://github.com/apache/maven/pull/1813


The 1970-01-01 may be threatened as "zero" in some systems. The PkZip
format epoch starts on 1980-01-01 and the date also maye have a
special meaning. So that the 1980-02-01 was chosen as a safe and easy
to remember.

The SOURCE_DATE_EPOCH is most of the time unnecessary and makes it
even more complicated to compare differences between different
versions.
We may need to have mtime updated for web assets to avoid caching
problems by Last-Modified but in most other cases this is not the
case.

Using the static reproducible build timestamp also simplifies the
build process and reduces dependency on the VCS.

So even if the repo build team does not endorse use of the 1980-02-01
it is still worth mentioning it on the Timestamps page.

What about making reproducible artifacts in other tools, like tar, zip, zstd?
Is there any place tracking adoption of the reproducible build by them?