Fix #72471: Docs to explain how adding/subtracting dates work

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@339459 c90b9560-bf6c-de11-be94-00142212c4b1

reference/datetime/book.xml

@@ -41,6 +41,7 @@
  
  &reference.datetime.setup;
  &reference.datetime.constants;
+ &reference.datetime.examples;
  
  &reference.datetime.datetime;
  &reference.datetime.datetimeimmutable;

reference/datetime/examples.xml

@@ -0,0 +1,126 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- $Revision$Id$ $ -->
+
+<chapter xml:id="datetime.examples" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
+ &reftitle.examples;
+
+ <section xml:id="datetime.examples-arithmetic">
+  <title>DateTime Arithmetic</title>
+  <para>
+   The following examples show some pitfalls of DateTime arithmetic with regard
+   to DST transitions and months having different numbers of days.
+  </para>
+  <para>
+   <example>
+    <title>DateTime::add/sub add intervals which cover elapsed time</title>
+    <simpara>
+        Adding PT24H over a DST transition will appear to add 23/25 hours (for
+        most timezones).
+    </simpara>
+    <programlisting role="php">
+<![CDATA[
+<?php
+$dt = new DateTime("2015-11-01 00:00:00", new DateTimeZone("America/New_York"));
+echo "Start: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
+$dt->add(new DateInterval("PT3H"));
+echo "End:   ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
+?>
+]]>
+    </programlisting>
+    &example.outputs;
+    <screen>
+<![CDATA[
+Start: 2015-11-01 00:00:00 -04:00
+End:   2015-11-01 02:00:00 -05:00
+]]>
+    </screen>
+   </example>
+  </para>
+  <para>
+   <example>
+    <title>DateTime::modify and strtotime increment or decrement individual component values</title>
+    <simpara>
+     Adding +24 hours over a DST transition will add exactly 24 hours as seen in
+     the date/time string (unless the start or end time is on a transition
+     point).
+    </simpara>
+    <programlisting role="php">
+<![CDATA[
+<?php
+$dt = new DateTime("2015-11-01 00:00:00", new DateTimeZone("America/New_York"));
+echo "Start: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
+$dt->modify("+24 hours");
+echo "End:   ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
+?>
+]]>
+    </programlisting>
+    &example.outputs;
+    <screen>
+<![CDATA[
+Start: 2015-11-01 00:00:00 -04:00
+End:   2015-11-02 00:00:00 -05:00
+]]>
+    </screen>
+   </example>
+  </para>
+  <para>
+   <example>
+    <title>Adding or subtracting times can over- or underflow dates</title>
+    <simpara>
+     Like where January 31st + 1 month will result in March 2nd (leap year) or
+     3rd (normal year).
+    </simpara>
+    <programlisting role="php">
+<![CDATA[
+<?php
+echo "Normal year:\n"; // February has 28 days
+$dt = new DateTime("2015-01-31 00:00:00", new DateTimeZone("America/New_York"));
+echo "Start: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
+$dt->modify("+1 month");
+echo "End:   ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
+
+echo "Leap year:\n"; // February has 29 days
+$dt = new DateTime("2016-01-31 00:00:00", new DateTimeZone("America/New_York"));
+echo "Start: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
+$dt->modify("+1 month");
+echo "End:   ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
+?>
+]]>
+    </programlisting>
+    &example.outputs;
+    <screen>
+<![CDATA[
+Normal year:
+Start: 2015-01-31 00:00:00 -05:00
+End:   2015-03-03 00:00:00 -05:00
+Leap year:
+Start: 2016-01-31 00:00:00 -05:00
+End:   2016-03-02 00:00:00 -05:00
+]]>
+    </screen>
+   </example>
+  </para>
+ </section>
+
+</chapter>
+
+<!-- Keep this comment at the end of the file
+Local variables:
+mode: sgml
+sgml-omittag:t
+sgml-shorttag:t
+sgml-minimize-attributes:nil
+sgml-always-quote-attributes:t
+sgml-indent-step:1
+sgml-indent-data:t
+indent-tabs-mode:nil
+sgml-parent-document:nil
+sgml-default-dtd-file:"~/.phpdoc/manual.ced"
+sgml-exposed-tags:nil
+sgml-local-catalogs:nil
+sgml-local-ecat-files:nil
+End:
+vim600: syn=xml fen fdm=syntax fdl=2 si
+vim: et tw=78 syn=sgml
+vi: ts=1 sw=1
+-->