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.