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Diffstat (limited to 'tz-how-to.html')
| -rw-r--r-- | tz-how-to.html | 161 |
1 files changed, 92 insertions, 69 deletions
diff --git a/tz-how-to.html b/tz-how-to.html index 2e4842d..bf3e86f 100644 --- a/tz-how-to.html +++ b/tz-how-to.html @@ -1,12 +1,16 @@ -<!DOCTYPE html - PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> -<html> +<!DOCTYPE html> +<html lang="en"> <head> <title>How to Read the tz Database</title> -<meta http-equiv="Content-type" content='text/html; charset="UTF-8"'> -<style type="text/css"> +<meta charset="UTF-8"> +<style> pre {margin-left: 2em; white-space: pre-wrap;} +pre.td {margin-left: 0;} +td {text-align: center;} +table {border: 1px outset;} +th, td {border: 1px inset;} +table.rule {border: none; margin: auto;} +td.footnote {text-align: left;} </style> </head> <body> @@ -31,14 +35,16 @@ about the zones.</p> for Chicago (from the <code>northamerica</code> file in the <code>data</code> subdirectory):</p> -<table border="1"> +<table> <tr> <th colspan="6">From the Source File</th> </tr> <tr> - <td colspan="6" align="center"><table><tr><td> -<pre> -#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER + <td colspan="6"> + <table class="rule"> + <tr><td style="border:none;text-align:left"> +<pre class="td"> +#Rule NAME FROM TO - IN ON AT SAVE LETTER Rule Chicago 1920 only - Jun 13 2:00 1:00 D Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D @@ -58,49 +64,58 @@ Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S <th>At</th> <th>Action</th> </tr> -<tr align="center"> +<tr> <td colspan="2">1920 only</td> <td colspan="2">June 13<small><sup>th</sup></small></td> <td rowspan="6">02:00 local</td> <td>go to daylight saving time</td> </tr> -<tr align="center"> +<tr> <td>1920</td> <td>1921</td> <td rowspan="5">last Sunday</td> <td>in October</td> <td>return to standard time</td> </tr> -<tr align="center"> +<tr> <td colspan="2">1921 only</td> <td>in March</td> <td rowspan="2">go to daylight saving time</td> </tr> -<tr align="center"> +<tr> <td rowspan="2">1922</td> <td>1966</td> <td>in April</td> </tr> -<tr align="center"> +<tr> <td>1954</td> <td>in September</td> <td rowspan="2">return to standard time</td> </tr> -<tr align="center"> +<tr> <td>1955</td> <td>1966</td> <td>in October</td> </tr> </table> -<p>We’ll basically just ignore the <code>TYPE</code> column. -In the 2007j release, the most recent as of this writing, the -<code>TYPE</code> column never contains anything but a hyphen, -a kind of null value. (From the description in <code>zic.8.txt</code>, -this appears to be a mechanism for removing years from a set -in some localizable way. It’s used in the file, <code>pacificnew</code>, -to determine whether a given year will have a US presidential election; -but everything related to that use is commented out.) +<p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the +first and last calendar years defining a contiguous range over which a specific +Rule line is to apply. The keyword <code>only</code> can be used in the +<code>TO</code> field to repeat the value of the <code>FROM</code> field in the +event that a rule should only apply to a single year. Often, the keyword +<code>max</code> is used to extend a rule’s application into the +indefinite future; it is a platform-agnostic stand-in for the largest +representable year. + +<p>The next column, <code>-</code>, is reserved; for compatibility with earlier +releases, it always contains a hyphen, which acts as a kind of null value. +Prior to the 2020b release, it was called the <code>TYPE</code> field, though +it was never used in the main data. An obsolescent supplementary file used the +field as a proof-of-concept to allow <code>zic</code> to apply a given Rule +line only to certain “types” of years within the specified range as +dictated by the output of a separate script, such as: only years which would +have a US presidential election, or only years which wouldn’t. <p>The <code>SAVE</code> column contains the local (wall clock) offset from local standard time. @@ -141,14 +156,16 @@ time changed in 1955. Got it?</p> <p>OK, now for the somewhat more interesting “US” rules:</p> -<table border="1"> +<table> <tr> <th colspan="6">From the Source File</th> </tr> <tr> - <td colspan="6" align="center"><table><tr><td> -<pre> -#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S + <td colspan="6"> + <table class="rule"> + <tr><td style="border:none;text-align:left"> +<pre class="td"> +#Rule NAME FROM TO - IN ON AT SAVE LETTER/S Rule US 1918 1919 - Mar lastSun 2:00 1:00 D Rule US 1918 1919 - Oct lastSun 2:00 0 S Rule US 1942 only - Feb 9 2:00 1:00 W # War @@ -175,7 +192,7 @@ Rule US 2007 max - Nov Sun>=1 2:00 0 S <th>At</th> <th>Action</th> </tr> -<tr align="center"> +<tr> <td rowspan="2">1918</td> <td rowspan="2">1919</td> <td rowspan="2">last Sunday</td> @@ -183,16 +200,16 @@ Rule US 2007 max - Nov Sun>=1 2:00 0 S <td rowspan="3">02:00 local</td> <td>go to daylight saving time</td> </tr> -<tr align="center"> +<tr> <td>in October</td> <td>return to standard time</td> </tr> -<tr align="center"> +<tr> <td colspan="2">1942 only</td> <td colspan="2">February 9<small><sup>th</sup></small></td> <td>go to “war time”</td> </tr> -<tr align="center"> +<tr> <td colspan="2" rowspan="2">1945 only</td> <td colspan="2">August 14<small><sup>th</sup></small></td> <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> @@ -201,47 +218,47 @@ Rule US 2007 max - Nov Sun>=1 2:00 0 S clocks don’t change </td> </tr> -<tr align="center"> +<tr> <td colspan="2">September 30<small><sup>th</sup></small></td> <td rowspan="9">02:00 local</td> <td rowspan="2">return to standard time</td> </tr> -<tr align="center"> +<tr> <td rowspan="2">1967</td> <td>2006</td> <td rowspan="2">last Sunday</td> <td>in October</td> </tr> -<tr align="center"> +<tr> <td>1973</td> <td>in April</td> <td rowspan="6">go to daylight saving time</td> </tr> -<tr align="center"> +<tr> <td colspan="2">1974 only</td> <td colspan="2">January 6<small><sup>th</sup></small></td> </tr> -<tr align="center"> +<tr> <td colspan="2">1975 only</td> <td colspan="2">February 23<small><sup>rd</sup></small></td> </tr> -<tr align="center"> +<tr> <td>1976</td> <td>1986</td> <td>last Sunday</td> <td rowspan="2">in April</td> </tr> -<tr align="center"> +<tr> <td>1987</td> <td>2006</td> <td>first Sunday</td> </tr> -<tr align="center"> +<tr> <td rowspan="2">2007</td> <td rowspan="2">present</td> <td colspan="2">second Sunday in March</td> </tr> -<tr align="center"> +<tr> <td colspan="2">first Sunday in November</td> <td>return to standard time</td> </tr> @@ -307,13 +324,15 @@ rule, so there should be no change.</li> <p>OK, now let’s look at a Zone record:</p> -<table border="1"> +<table> <tr> <th colspan="5">From the Source File</th> </tr> <tr> - <td colspan="6" align="center"><table><tr><td> -<pre> + <td colspan="5"> + <table class="rule"> + <tr><td style="border:none;text-align:left"> +<pre class="td"> #Zone NAME STDOFF RULES FORMAT [UNTIL] Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 -6:00 US C%sT 1920 @@ -341,49 +360,49 @@ Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 <th>Date</th> <th>Time</th> </tr> -<tr align="center"> +<tr> <td>−5:50:36</td> <td>not observed</td> <td>LMT</td> <td>1883-11-18</td> <td>12:09:24</td> </tr> -<tr align="center"> +<tr> <td rowspan="2">−6:00:00</td> <td>US rules</td> <td rowspan="2">CST or CDT</td> <td>1920-01-01</td> <td>00:00:00</td> </tr> -<tr align="center"> +<tr> <td>Chicago rules</td> <td>1936-03-01</td> <td rowspan="2">02:00:00</td> </tr> -<tr align="center"> +<tr> <td>−5:00:00</td> <td>not observed</td> <td>EST</td> <td>1936-11-15</td> </tr> -<tr align="center"> +<tr> <td rowspan="4">−6:00:00</td> <td>Chicago rules</td> <td>CST or CDT</td> <td>1942-01-01</td> <td rowspan="3">00:00:00</td> </tr> -<tr align="center"> +<tr> <td>US rules</td> <td>CST, CWT or CPT</td> <td>1946-01-01</td> </tr> -<tr align="center"> +<tr> <td>Chicago rules</td> <td rowspan="2">CST or CDT</td> <td>1967-01-01</td> </tr> -<tr align="center"> +<tr> <td>US rules</td> <td colspan="2">—</td> </tr> @@ -548,14 +567,16 @@ the true offset is undefined. <p>As a final example, here’s the complete history for Hawaii:</p> -<table border="1"> +<table> <tr> <th colspan="6">Relevant Excerpts from the US Rules</th> </tr> <tr> - <td colspan="6" align="center"><table><tr><td> -<pre> -#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S + <td colspan="6"> + <table class="rule"> + <tr><td style="border:none;text-align:left"> +<pre class="td"> +#Rule NAME FROM TO - IN ON AT SAVE LETTER/S Rule US 1918 1919 - Oct lastSun 2:00 0 S Rule US 1942 only - Feb 9 2:00 1:00 W # War Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace @@ -567,8 +588,10 @@ Rule US 1945 only - Sep lastSun 2:00 0 S <th colspan="6">The Zone Record</th> </tr> <tr> - <td colspan="6" align="center"><table><tr><td> -<pre> + <td colspan="6"> + <table class="rule"> + <tr><td style="border:none;text-align:left"> +<pre class="td"> #Zone NAME STDOFF RULES FORMAT [UNTIL] Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 -10:30 - HST 1933 Apr 30 2:00 @@ -593,7 +616,7 @@ Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 <th>Date</th> <th>Time</th> </tr> -<tr align="center"> +<tr> <td>−10:31:26</td> <td>—</td> <td>LMT</td> @@ -601,7 +624,7 @@ Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 <td>1896-01-13</td> <td>12:00</td> </tr> -<tr align="center"> +<tr> <td>−10:30</td> <td>+0:01:26</td> <td>HST</td> @@ -609,7 +632,7 @@ Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 <td>1933-04-30</td> <td>02:00</td> </tr> -<tr align="center"> +<tr> <td>−9:30</td> <td>+1:00</td> <td>HDT</td> @@ -617,7 +640,7 @@ Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 <td>1933-05-21</td> <td>12:00</td> </tr> -<tr align="center"> +<tr> <td>−10:30¹</td> <td>−1:00¹</td> <td>HST¹</td> @@ -625,7 +648,7 @@ Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 <td>1942-02-09</td> <td>02:00</td> </tr> -<tr align="center"> +<tr> <td rowspan="2">−9:30</td> <td>+1:00</td> <td>HWT</td> @@ -633,38 +656,38 @@ Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 <td>1945-08-14</td> <td>13:30²</td> </tr> -<tr align="center"> +<tr> <td>0</td> <td>HPT</td> <td>Hawaii peace time</td> <td>1945-09-30</td> <td rowspan="2">02:00</td> </tr> -<tr align="center"> +<tr> <td>−10:30</td> <td>−1:00</td> <td rowspan="2">HST</td> <td rowspan="2">Hawaii standard time</td> <td>1947-06-08</td> </tr> -<tr align="center"> +<tr> <td>−10:00³</td> <td>+0:30³</td> <td colspan="2">—</td> </tr> <tr> - <td colspan="6"> + <td colspan="6" class="footnote"> ¹Switching to US rules…most recent transition (in 1919) was to standard time </td> </tr> <tr> - <td colspan="6"> + <td colspan="6" class="footnote"> ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> + (−9:30) = 13:30 local </td> </tr> <tr> - <td colspan="6"> + <td colspan="6" class="footnote"> ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, the civil time in Hawaii has been <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> |