Promote the default reproducible build date

[Sergey Ponomarev]

The 1980-02-01 was chosen as a default reproducible date in many tools
like Gradle and Maven (any others?).
Could you mention it on the page
https://reproducible-builds.org/docs/timestamps/
We need to put rationale why the date was chosen to be a default.

Some other tools like archivers may need a simpler way to generate
repo archives without timestamps.
For example today I wanted to do this for a tar command and found that
this is not so easy to do. Search shows the StackOverflow "How to
create a tar file that omits timestamps for its contents?" thread the
points to the article official reproducible-builds article "Archive
metadata" that proposes the long command:

tar --sort=name \
      --mtime="@${SOURCE_DATE_EPOCH}" \
      --owner=0 --group=0 --numeric-owner \
      --pax-option=exthdr.name=%d/PaxHeaders/%f,delete=atime,delete=ctime \
      -cf product.tar build

I have a few propositions:
1. If the SOURCE_DATE_EPOCH var is empty use 1980-02-01 by default
--mtime="@${SOURCE_DATE_EPOCH:-318211200}"
2. Maybe we can propose a patch to the gnu tar to read the
SOURCE_DATE_EPOCH env and use it by default?
3. Maybe we can propose a patch to the gnu tar to have a short option
--reproducible that will set other options --mtime --owner --group
--pax-option to the needed values.

We should make this easier to use. Other tools like zip, zstd may also
need for same behaviour.