[tz] [PROPOSED] Shanks is likely wrong about Maputo switch to CAT

[Paul Eggert]

* NEWS: Mention the change, and leave room for future Portugal fixes.
* africa (Africa/Maputo): Tweak LMT to match London Gazette,
and change CAT transition date to a guess of 1909.
---
 NEWS   |  4 ++++
 africa | 19 ++++++++++++++-----
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/NEWS b/NEWS
index 25d939f1..0d6a1513 100644
--- a/NEWS
+++ b/NEWS
@@ -4,6 +4,7 @@ Unreleased, experimental changes
 
   Briefly:
     The main data form now uses %z.
+    Improve historical data for Portugal and possessions.
 
   Changes to data
 
@@ -15,6 +16,9 @@ Unreleased, experimental changes
     the represented data: the generated TZif files are unchanged.
     Rearguard form still avoids %z, to support obsolescent parsers.
 
+    Some transitions for Portugal and possessions have been changed.
+    This affects timestamps for Africa/Maputo before 1909.
+
     Asia/Almaty has been removed from zonenow.tab as it now agrees
     with Asia/Tashkent for future timestamps, due to Kazakhstan's
     2024-02-29 time zone change.  Similarly, America/Scoresbysund
diff --git a/africa b/africa
index a731dbee..695d66ed 100644
--- a/africa
+++ b/africa
@@ -1101,13 +1101,22 @@ Zone Africa/El_Aaiun	-0:52:48 -	LMT	1934 Jan # El Aaiún
 # Zambia
 # Zimbabwe
 #
-# Shanks gives 1903-03-01 for the transition to CAT.
-# Perhaps the 1911-05-26 Portuguese decree
-# https://dre.pt/pdf1sdip/1911/05/12500/23132313.pdf
-# merely made it official?
+# From Paul Eggert (2024-04-09):
+# The London Gazette, 1903-04-03, page 2245, says that
+# as of 1903-03-03 a time ball at the port of Lourenço Marques
+# (as Maputo was then called) was dropped daily at 13:00:00 LMT,
+# corresponding to 22:49:41.7 GMT; round this to UT+2:10:18.
+# Conversely, the newspaper South Africa, 1909-02-09, page 321,
+# says the port had just installed an apparatus that communicated
+# "from the controlling clock in the new Observatory at Reuben Point ...
+# exact mean South African time, i.e., 30 deg., or 2 hours East of Greenwich".
+# Although Shanks gives 1903-03-01 for the transition to CAT,
+# evidently the port transitioned to CAT after 1903-03-03 but before
+# the Portuguese legal transition of 1912-01-01 (see Europe/Lisbon commentary).
+# For lack of better info, list 1909 as the transition date.
 #
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
-Zone	Africa/Maputo	2:10:20 -	LMT	1903 Mar
+Zone	Africa/Maputo	2:10:18 -	LMT	1909
 			2:00	-	CAT
 
 # Namibia
-- 
2.44.0