Nix sets everything to the epoch, although I believe Debian's approach is to just use the date of the newest file in the dsc tarballs.

Nix can also set it to things other than 0; I think my favorite is to set it by the time of the commit from which you're building.

Which is also used when the contents of a derivation will be included in a zip file. The Unix epoch is about a decade older than the zip epoch.

Debian's approach is actually to use the date specified in the top entry in the debian/changelog file. That's more transparent and resilient than any mtime.

Strangely enough, sometimes using the epoch can expose bugs in libraries (etc.) when running or building in a timezone west of Greenwich due to the negative time offset taking time "below" zero.