DateTime + 12 hours showing same DateTime. Why? : <p>When I am increasing DateTime value by any hours, the result is OKAY, but when I increase it by 12 hours, it is not increasing. 
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Please see the following code for details:
<br/><br/><br/><br/></p>

<pre><code>$creation_date = new DateTime('2016-09-07 06:00:00', new DateTimeZone('Asia/Kolkata'));
$expiration_date = new DateTime('2016-09-07 06:00:00', new DateTimeZone('Asia/Kolkata'));
</code></pre>

<p>When I increase the <code>$expiration_date</code> variable by 1 hour, 3 hour, 8 hour, 24 hours etc., the result is perfect. For example,</p>

<p>Case 1:<br/>    </p>

<pre><code>$expiration_date-&gt;add(new DateInterval('PT1H'));
echo "Creation Date: ".$creation_date-&gt;format('Asia/Kolkata')."&lt;br/&gt;Expiration Date: ".$expiration_date-&gt;format('Asia/Kolkata');
</code></pre>

<p>Result 1:<br/><br>
Creation Date: 2016-09-07 06:00:00<br/>
Expiration Date: 2016-09-07 07:00:00</p>

<p>Case 2:<br/>    </p>

<pre><code>$expiration_date-&gt;add(new DateInterval('PT3H'));
echo "Creation Date: ".$creation_date-&gt;format('Asia/Kolkata')."&lt;br/&gt;Expiration Date: ".$expiration_date-&gt;format('Asia/Kolkata');
</code></pre>

<p>Result 2:<br/>
Creation Date: 2016-09-07 06:00:00<br/>
Expiration Date: 2016-09-07 09:00:00</p>

<p>Case 3:<br/>    </p>

<pre><code>$expiration_date-&gt;add(new DateInterval('PT8H'));
echo "Creation Date: ".$creation_date-&gt;format('Asia/Kolkata')."&lt;br/&gt;Expiration Date: ".$expiration_date-&gt;format('Asia/Kolkata');
</code></pre>

<p>Result 3:<br/>
Creation Date: 2016-09-07 06:00:00<br/>
Expiration Date: 2016-09-07 02:00:00</p>

<p>Case 4:<br/>    </p>

<pre><code>$expiration_date-&gt;add(new DateInterval('PT24H'));
echo "Creation Date: ".$creation_date-&gt;format('Asia/Kolkata')."&lt;br/&gt;Expiration Date: ".$expiration_date-&gt;format('Asia/Kolkata');
</code></pre>

<p>Result 4:<br/>
Creation Date: 2016-09-07 06:00:00<br/>
Expiration Date: 2016-09-08 06:00:00</p>

<p><strong>But when I increase the <code>$expiration_date</code> variable by <em>12 hours</em>, the date is not getting increased!<br/><br/>They are showing the same datetime!</strong></p>

<p>Case 5:<br/>    </p>

<pre><code>$expiration_date-&gt;add(new DateInterval('PT12H'));
echo "Creation Date: ".$creation_date-&gt;format('Asia/Kolkata')."&lt;br/&gt;Expiration Date: ".$expiration_date-&gt;format('Asia/Kolkata');
</code></pre>

<p>Result 5:<br/>
Creation Date: 2016-09-07 06:00:00<br/>
Expiration Date: 2016-09-07 06:00:00
<br/><br/><br/><br/>
What am I doing wrong?</p>
