Fix incorrect timezones for London and Paris

Also, replace obsolete city name "Bangalore" with "Delhi",
and improve doc strings.
* lisp/time.el (zoneinfo-style-world-list)
(legacy-style-world-list): Evidently "Bangalore" was chosen for
Asia/Kolkata to avoid confusion about "Calcutta" vs "Kolkata".
However, it has similar confusion with "Bengaluru".
Use "Delhi" instead, as Delhi is larger, its name has not changed
for some time, and proposals to change its name have so far failed.
See: Sharma MS. Vijay Goel wants Delhi renamed as Dilli, but historians
say it will only spell trouble. Times of India. 2019-07-25.
(zoneinfo-style-world-list): Update doc string. “Posix-style” was
ambiguous as TZDB-style strings are specified only by POSIX.1-2024,
so use “TZDB-style” instead. Say “AREA/LOCATION” to be consistent.
If AREA/LOCATION is unsupported, Emacs signals an error on NetBSD
so don’t imply that some other timezone will be used.
(legacy-style-world-list): Remove ("GMT0BST" "London") and ("CET-1CDT"
"Paris"). These are wrong because neither London nor Paris follow
US DST rules.  Instead, use ("GMT0BST,M3.5.0/1,M10.5.0" "London")
and ("CET-1CEST,M3.5.0,M10.5.0/3" "Paris") if they work, and omit
London and Paris entries otherwise.

1 files changed, 23 insertions, 13 deletions

diff --git a/lisp/time.el b/lisp/time.el
index 517eea20ad1..ecacaf08aa4 100644
--- a/lisp/time.el
+++ b/lisp/time.el
@@ -452,7 +452,7 @@ runs the normal hook `display-time-hook' after each update."
     ("America/New_York" "New York")
     ("Europe/London" "London")
     ("Europe/Paris" "Paris")
-    ("Asia/Kolkata" "Bangalore")
+    ("Asia/Kolkata" "Delhi")
     ("Asia/Tokyo" "Tokyo"))
   "Alist of zoneinfo-style time zones and places for `world-clock'.
 Each element has the form (TIMEZONE LABEL).
@@ -461,27 +461,37 @@ the name of a region -- a continent or ocean, and LOCATION is the name
 of a specific location, e.g., a city, within that region.
 LABEL is a string to display as the label of that TIMEZONE's time.
 
-This option has effect only on systems that support Posix-style
-zoneinfo files specified as CONTINENT/CITY.  In particular,
+This option has effect only on systems that support TZDB-style
+TZ strings specified as AREA/LOCATION.  In particular,
 MS-Windows doesn't support that; use `legacy-style-world-list' instead.
-Also, AREA/LOCATION must specify a zoneinfo file installed on your
-system, otherwise what timezone will be used depends on the underlying
-OS and library."
+Also, AREA/LOCATION must specify a timezone supported by your system,
+otherwise the behavior depends on the underlying OS and library."
   :type '(repeat (list string string))
   :version "23.1")
 
 (defcustom legacy-style-world-list
-  '(("PST8PDT" "Seattle")
-    ("EST5EDT" "New York")
-    ("GMT0BST" "London")
-    ("CET-1CDT" "Paris")
-    ("IST-5:30" "Bangalore")
-    ("JST-9" "Tokyo"))
+  ;; This list is for circa 2025 timekeeping, and will need changes
+  ;; if any of these locations change names, time zones, or DST rules.
+  (append
+   ;; These entries assume the platform's default DST rules are for the US,
+   ;; which is true of all known platforms that use this variable.
+   '(("PST8PDT" "Seattle")
+     ("EST5EDT" "New York"))
+
+   ;; Use POSIX.1-2017 style TZ if supported; otherwise, give up on London
+   ;; and Paris as "GMT0BST" and "CET-1CDT" would use incorrect DST.
+   (when (string-equal (format-time-string "%z" 0 "XXX0YYY,M9.5.0/1,M4.1.0/3")
+                       "+0100")
+     '(("GMT0BST,M3.5.0/1,M10.5.0" "London")
+       ("CET-1CEST,M3.5.0,M10.5.0/3" "Paris")))
+
+   '(("IST-5:30" "Delhi")
+     ("JST-9" "Tokyo")))
   "Alist of traditional-style time zones and places for `world-clock'.
 Each element has the form (TIMEZONE LABEL).
 TIMEZONE should be a string of the form:
 
-     std[+|-]offset[dst[offset][,date[/time],date[/time]]]
+     std[+|-]offset[dst[[+|-]offset][,date[/time],date[/time]]]
 
 See the documentation of the TZ environment variable on your system,
 for more details about the format of TIMEZONE.