[Git][reproducible-builds/reproducible-website][master] 2 commits: Revert "Add a note re. TZ=UTC vs. TZ=UTC0"

[Chris Lamb (@lamby)]

Chris Lamb pushed to branch master at Reproducible Builds / reproducible-website


Commits:
0ca7cc78 by Chris Lamb at 2026-09-30T08:56:02-07:00
Revert "Add a note re. TZ=UTC vs. TZ=UTC0"

This reverts commit adf38b870244e4cc9bed50c1b5c70a7a647ac66a.

- - - - -
ae8f4da5 by Chris Lamb at 2026-09-30T08:56:02-07:00
Correct TZ=UTC validity. Thanks, Stephen Kitt! (skitt).

- - - - -


1 changed file:

- _docs/timezones.md


Changes:

=====================================
_docs/timezones.md
=====================================
@@ -43,18 +43,22 @@ Tue, 19 Jan 2038 03:14:07 +0000
 {% endhighlight %}
 </div>
 
-Note that `TZ=UTC` is *technically* invalid, although it will work on all
-observed `libc` implementations. [Chapter 8 of the POSIX standard](https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html)
-states that the `TZ` format should be:
-
-```
-std offset [dst [offset][, rule]]
-```
-
-… where `std` and `offset` are required. In particular, the definition of
-`offset` states "The hour (*hh*) shall be required and may be a single digit. The
-*offset* following *std* shall be required." Therefore `TZ=UTC` is technically
-invalid as it misses the required offset; `TZ=UTC0` would be compliant.
+Note that both `TZ=UTC` and `TZ=UTC0` styles are valid.
+[Chapter 8 of the POSIX standard](https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html)
+specifies a number of formats:
+
+* `:characters`
+* `std offset dst offset, rule`
+* "a format specifying a geographical timezone or a special timezone"
+
+`UTC0` matches the second one and `UTC` matches the third.
+
+The main benefit of `UTC0` is that it is fully specified in POSIX; on any
+POSIX-compliant system, `TZ=UTC` and `TZ=UTC0` will mean the same thing.
+`TZ=UTC` relies on the implementation, however; typically it would use the IANA
+database (which is mentioned in POSIX), but there's nothing stopping an
+implementation from interpreting `TZ=UTC` as the timezone of a putative “United
+Transcendental Countries” for example.
 
 A related concern is for formats which don't contain timezone
 information. Zip archives are a good example: the same timezone must



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