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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<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>Date/Time Arithmetic</title>
<para>
The following examples show some pitfalls of Date/Time arithmetic with regard
to DST transitions and months having different numbers of days.
</para>
<para>
<example>
<title>DateTimeImmutable::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 DateTimeImmutable("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 = $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>DateTimeImmutable::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 DateTimeImmutable("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 = $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 DateTimeImmutable("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 = $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 DateTimeImmutable("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 = $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>
<simpara>
To get the last day of the next month (i.e. to prevent the overflow),
the <literal>last day of</literal> format is available.
</simpara>
<programlisting role="php">
<![CDATA[
<?php
echo "Normal year:\n"; // February has 28 days
$dt = new DateTimeImmutable("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 = $dt->modify("last day of next month");
echo "End: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
echo "Leap year:\n"; // February has 29 days
$dt = new DateTimeImmutable("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 = $dt->modify("last day of next 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: 2015-02-28 00:00:00 -05:00
Leap year:
Start: 2016-01-31 00:00:00 -05:00
End: 2016-02-29 00:00:00 -05:00
]]>
</screen>
</example>
</para>
</section>
</chapter>
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