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</div><hr /><div id="datetime.examples-arithmetic" class="section">
  <h2 class="title">DateTime Arithmetic</h2>
  <p class="para">
   The following examples show some pitfalls of DateTime arithmetic with regard
   to DST transitions and months having different numbers of days.
  </p>
  <p class="para">
   <div class="example" id="example-2736">
    <p><strong>Example #1 DateTime::add/sub add intervals which cover elapsed time</strong></p>
    <div class="example-contents"><p>
        Adding PT24H over a DST transition will appear to add 23/25 hours (for
        most timezones).
    </p></div>
    <div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br />$dt&nbsp;</span><span style="color: #007700">=&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTime</span><span style="color: #007700">(</span><span style="color: #DD0000">"2015-11-01&nbsp;00:00:00"</span><span style="color: #007700">,&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTimeZone</span><span style="color: #007700">(</span><span style="color: #DD0000">"America/New_York"</span><span style="color: #007700">));<br />echo&nbsp;</span><span style="color: #DD0000">"Start:&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">add</span><span style="color: #007700">(new&nbsp;</span><span style="color: #0000BB">DateInterval</span><span style="color: #007700">(</span><span style="color: #DD0000">"PT3H"</span><span style="color: #007700">));<br />echo&nbsp;</span><span style="color: #DD0000">"End:&nbsp;&nbsp;&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
    </div>

    <div class="example-contents"><p>The above example will output:</p></div>
    <div class="example-contents screen">
<div class="cdata"><pre>
Start: 2015-11-01 00:00:00 -04:00
End:   2015-11-01 02:00:00 -05:00
</pre></div>
    </div>
   </div>
  </p>
  <p class="para">
   <div class="example" id="example-2737">
    <p><strong>Example #2 DateTime::modify and strtotime increment or decrement individual component values</strong></p>
    <div class="example-contents"><p>
     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).
    </p></div>
    <div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br />$dt&nbsp;</span><span style="color: #007700">=&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTime</span><span style="color: #007700">(</span><span style="color: #DD0000">"2015-11-01&nbsp;00:00:00"</span><span style="color: #007700">,&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTimeZone</span><span style="color: #007700">(</span><span style="color: #DD0000">"America/New_York"</span><span style="color: #007700">));<br />echo&nbsp;</span><span style="color: #DD0000">"Start:&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">modify</span><span style="color: #007700">(</span><span style="color: #DD0000">"+24&nbsp;hours"</span><span style="color: #007700">);<br />echo&nbsp;</span><span style="color: #DD0000">"End:&nbsp;&nbsp;&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
    </div>

    <div class="example-contents"><p>The above example will output:</p></div>
    <div class="example-contents screen">
<div class="cdata"><pre>
Start: 2015-11-01 00:00:00 -04:00
End:   2015-11-02 00:00:00 -05:00
</pre></div>
    </div>
   </div>
  </p>
  <p class="para">
   <div class="example" id="example-2738">
    <p><strong>Example #3 Adding or subtracting times can over- or underflow dates</strong></p>
    <div class="example-contents"><p>
     Like where January 31st + 1 month will result in March 2nd (leap year) or
     3rd (normal year).
    </p></div>
    <div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br /></span><span style="color: #007700">echo&nbsp;</span><span style="color: #DD0000">"Normal&nbsp;year:\n"</span><span style="color: #007700">;&nbsp;</span><span style="color: #FF8000">//&nbsp;February&nbsp;has&nbsp;28&nbsp;days<br /></span><span style="color: #0000BB">$dt&nbsp;</span><span style="color: #007700">=&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTime</span><span style="color: #007700">(</span><span style="color: #DD0000">"2015-01-31&nbsp;00:00:00"</span><span style="color: #007700">,&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTimeZone</span><span style="color: #007700">(</span><span style="color: #DD0000">"America/New_York"</span><span style="color: #007700">));<br />echo&nbsp;</span><span style="color: #DD0000">"Start:&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">modify</span><span style="color: #007700">(</span><span style="color: #DD0000">"+1&nbsp;month"</span><span style="color: #007700">);<br />echo&nbsp;</span><span style="color: #DD0000">"End:&nbsp;&nbsp;&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /><br />echo&nbsp;</span><span style="color: #DD0000">"Leap&nbsp;year:\n"</span><span style="color: #007700">;&nbsp;</span><span style="color: #FF8000">//&nbsp;February&nbsp;has&nbsp;29&nbsp;days<br /></span><span style="color: #0000BB">$dt&nbsp;</span><span style="color: #007700">=&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTime</span><span style="color: #007700">(</span><span style="color: #DD0000">"2016-01-31&nbsp;00:00:00"</span><span style="color: #007700">,&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTimeZone</span><span style="color: #007700">(</span><span style="color: #DD0000">"America/New_York"</span><span style="color: #007700">));<br />echo&nbsp;</span><span style="color: #DD0000">"Start:&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">modify</span><span style="color: #007700">(</span><span style="color: #DD0000">"+1&nbsp;month"</span><span style="color: #007700">);<br />echo&nbsp;</span><span style="color: #DD0000">"End:&nbsp;&nbsp;&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
    </div>

    <div class="example-contents"><p>The above example will output:</p></div>
    <div class="example-contents screen">
<div class="cdata"><pre>
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
</pre></div>
    </div>
    <div class="example-contents"><p>
     To get the last day of the next month (i.e. to prevent the overflow),
     the <em>last day of</em> format is available as of PHP 5.3.0.
    </p></div>
    <div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br /></span><span style="color: #007700">echo&nbsp;</span><span style="color: #DD0000">"Normal&nbsp;year:\n"</span><span style="color: #007700">;&nbsp;</span><span style="color: #FF8000">//&nbsp;February&nbsp;has&nbsp;28&nbsp;days<br /></span><span style="color: #0000BB">$dt&nbsp;</span><span style="color: #007700">=&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTime</span><span style="color: #007700">(</span><span style="color: #DD0000">"2015-01-31&nbsp;00:00:00"</span><span style="color: #007700">,&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTimeZone</span><span style="color: #007700">(</span><span style="color: #DD0000">"America/New_York"</span><span style="color: #007700">));<br />echo&nbsp;</span><span style="color: #DD0000">"Start:&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">modify</span><span style="color: #007700">(</span><span style="color: #DD0000">"last&nbsp;day&nbsp;of&nbsp;next&nbsp;month"</span><span style="color: #007700">);<br />echo&nbsp;</span><span style="color: #DD0000">"End:&nbsp;&nbsp;&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /><br />echo&nbsp;</span><span style="color: #DD0000">"Leap&nbsp;year:\n"</span><span style="color: #007700">;&nbsp;</span><span style="color: #FF8000">//&nbsp;February&nbsp;has&nbsp;29&nbsp;days<br /></span><span style="color: #0000BB">$dt&nbsp;</span><span style="color: #007700">=&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTime</span><span style="color: #007700">(</span><span style="color: #DD0000">"2016-01-31&nbsp;00:00:00"</span><span style="color: #007700">,&nbsp;new&nbsp;</span><span style="color: #0000BB">DateTimeZone</span><span style="color: #007700">(</span><span style="color: #DD0000">"America/New_York"</span><span style="color: #007700">));<br />echo&nbsp;</span><span style="color: #DD0000">"Start:&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">modify</span><span style="color: #007700">(</span><span style="color: #DD0000">"last&nbsp;day&nbsp;of&nbsp;next&nbsp;month"</span><span style="color: #007700">);<br />echo&nbsp;</span><span style="color: #DD0000">"End:&nbsp;&nbsp;&nbsp;"</span><span style="color: #007700">,&nbsp;</span><span style="color: #0000BB">$dt</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">format</span><span style="color: #007700">(</span><span style="color: #DD0000">"Y-m-d&nbsp;H:i:s&nbsp;P"</span><span style="color: #007700">),&nbsp;</span><span style="color: #0000BB">PHP_EOL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
    </div>

    <div class="example-contents"><p>The above example will output:</p></div>
    <div class="example-contents screen">
<div class="cdata"><pre>
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
</pre></div>
    </div>
   </div>
  </p>
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