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.

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.

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.

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.