This does not trace things back far enough. The root is where IANA has long since segregated out a set of timezone file names into a "backward" collection: *

This does not trace things back far enough. The root is where IANA has long since segregated out a set of timezone file names into a "backward" collection: * https://data.iana.org/time-zones/tzdb/backward https://data.iana.org/time-zones/tzdb/backward If one traces references, one finds this connected bug on Launchpad. Amusingly to anyone who has ever seen these sweeping timezone database changes over the years, Launchpad marked it as "This bug affects 1 person.". * https://bugs.launchpad.net/ubuntu/+source/tzdata/+bug/2008076 https://bugs.launchpad.net/ubuntu/+source/tzdata/+bug/200807... The rules for the "backward" file are here: * https://data.iana.org/time-zones/theory.html#naming https://data.iana.org/time-zones/theory.html#naming All of the US/* timezone names, such as US/Pacific here, have been backwards compatibility measures in place for the whole of the 21st century and some of the late 20th. The Olson database in the 1980s (mod.sources v08i085, comp.sources.unix v14i030) used these names. But the naming scheme changed somewhen in the 1990s to a continent/city and ocean/city form and backwards compatibility with the old names has been preserved ever since by the "backward" file. * https://groups.google.com/g/comp.unix.solaris/c/ON_MPZxVdv0/m/Zhx8i142qLIJ https://groups.google.com/g/comp.unix.solaris/c/ON_MPZxVdv0/... Debian has moved into a non-depended-upon package a backwards compatibility measure that is as old as Debian itself.