[tz] Dealing with Pre-1970 Data

Paul Eggert eggert at cs.ucla.edu
Sat Aug 31 00:36:37 UTC 2013

Guy Harris wrote:
> If we're obliged to leave them in the tzdb for backwards compatibility purposes, we should:
> 	accompany them with a disclaimer that they're not actually meaningful

That sounds reasonable.  Here's a draft of a disclaimer,
along with a pointer to a discussion of how little
we know even about solar time if we go back far enough,
and if it weren't for those amazing Babylonian astronomers
we'd know even less.  I've pushed this into the
experimental repository.

>From 5e8489b16dfe4cf7493ad7a3578d90656236d310 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert at cs.ucla.edu>
Date: Fri, 30 Aug 2013 16:43:57 -0700
Subject: [PATCH] * Theory: Describe LMT better.

Following a suggestion by Guy Harris in
 Theory | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/Theory b/Theory
index b4bd4c2..580b548 100644
--- a/Theory
+++ b/Theory
@@ -224,7 +224,23 @@ could misbehave if data were omitted for pre-1970 transitions.
 However, the database is not designed for and does not suffice for
 applications requiring accurate handling of all past times everywhere,
 as it would take far too much effort and guesswork to record all
-details of pre-1970 civil timekeeping.
+details of pre-1970 civil timekeeping.  The pre-1970 data in this
+database covers only a tiny sliver of how clocks actually behaved;
+the vast majority of the necessary information was lost or never
+recorded, and much of what little remains is fabricated.
+Local mean time (LMT) offsets are recorded in the database only
+because the format requires an offset.  They should not be considered
+meaningful, and should not prompt creation of zones merely because two
+locations differ in LMT.  Historically, not only did different
+locations in the same zone typically use different LMT offsets, often
+different people in the same location maintained mean-time clocks that
+differed significantly, and many people used solar or some other time
+instead of mean time.  As for leap seconds, we don't know the history
+of earth's rotation accurately enough to map SI seconds to historical
+solar time to more than about one-hour accuracy; see Stephenson FR
+(2003), Historical eclipses and Earth's rotation, A&G 44: 2.22-2.27
 As noted in the README file, the tz database is not authoritative
 (particularly not for pre-1970 time stamps), and it surely has errors.
@@ -384,8 +400,11 @@ in decreasing order of importance:
 			name identifying each zone and append 'T', 'ST', etc.
 			as before; e.g. 'VLAST' for VLAdivostok Summer Time.
-	Use UTC (with time zone abbreviation "zzz") for locations while
-		uninhabited.  The "zzz" mnemonic is that these locations are,
+	Use 'LMT' for local mean time of locations before the introduction
+		of standard time; see "Scope of the tz database".
+	Use UTC (with time zone abbreviation 'zzz') for locations while
+		uninhabited.  The 'zzz' mnemonic is that these locations are,
 		in some sense, asleep.
 Application writers should note that these abbreviations are ambiguous

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