<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<p><br>
</p>
<div class="moz-cite-prefix">On 03.06.20 12:17, Clive D.W. Feather
wrote:<br>
</div>
<blockquote type="cite" cite="mid:20200603101748.GD41274@davros.org">
<pre class="moz-quote-pre" wrap="">Alois Treindl said:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap=""> I have analyzed to what extent TZ database represents the time zone
information correctly, for these data.
I think the results are of some interest for the TZ community.
The overall sums are:
tz_count 24'674'767 data records, 100%
tz_irange 23'675'636 in time range and region which TZ covers
correctly, 96%
tz_good 736'769 in time range and region where TZ is unreliable,
but correct, 3%
tz_bad 262'362 in time range and region where TZ database gives
false result. 1%
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
Can you please explain how you've done this analysis. In particular, how do
you know that TZ is "unreliable, but correct" or gives a bad result?
</pre>
</blockquote>
<p>As I said, we have other sources which we use besides TZ data.
These sources are used for the pre-1970 cases where we know that
TZ data is incomplete.<br>
</p>
<p>I give you an example:<br>
</p>
For Germany TZ data uses Berlin. But Berlin had double summer time
in 1945, which the western occupation zones (what later became
federal republic of Germany) did not have.<br>
Europe/Berlin does not represent the whole of Germany well, before
1946.<br>
<br>
There are also some minor differences in the late 19th century, when
different provinces switched from local mean time to Central
European time.<br>
There are other complications, not represented by TZ database.<br>
For example in the 1920s France coccupied the Rhineland and Ruhr
area, and west of the Rhine French time (UT+0) was used.<br>
After 1st world war, parts of Prussia fell to Poland.<br>
After 2nd world war, more parts of Prussia fell to Poland, and
Königsberg province (Kaliningrad) fell to Russia. <br>
These all have consequences for correct pre-1970 time zone history.<br>
<br>
This means in our system that for West Germany, TZ data is
unreliable before 1946, and we do not use it, but use other tables.
<p>This incorrectness of TZ database for Germany applies only to
those few months on 1945. But we say 'West Germany before 1946, do
not use TZ database'.</p>
<p>the results for Germany are:<br>
</p>
<p>GER 1202363 95.4 4.4 0.2 Germany</p>
<p>this means: we have 1'202'363 cases.</p>
<p>95.4% of these fall either after 1 Jan 1946, or are in area
described correctly by Europe/Berlin. For these we use TZ
database.<br>
4.6% of cases do not fall into this category, and we do not use TZ
databases for them.</p>
<p>If we would use TZ database for these 4.6% cases, 4.4% would come
out correct anyway, but 0.2% would come out bad.<br>
<br>
This are 0.2 % of those data records our users have entered into
our database, and used out 'automatic time zone' setting. They
could have used 'manual time zone', and I have ignored those for
the statistics.<br>
</p>
</body>
</html>