Description:
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When adding a DateInterval to a DateTime object, it adds unexpected amounts of seconds. 

For the following time frames, adding DateInterval doesn't work correctly (my PHP version 5.4.27)

// PT1M – adding 1 minute. Expected timestamp differential : 60 seconds

1414907940 - 1414911539 ( 2014-11-02 05:59:00 - 2014-11-02 06:58:59 )
test script result: -3540
1414911540 - 1414911599 ( 2014-11-02 06:59:00 - 2014-11-02 06:59:59 )
test script result: -7140
1446357540 - 1446361139 ( 2015-11-01 05:59:00 - 2015-11-01 06:58:59 )
test script result: -3540
1446361140 - 1446361199 ( 2015-11-01 06:59:00 - 2015-11-01 06:59:59 )
test script result: -7140

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// PT1H -  adding 1 hour. Expected timestamp differential : 3600 seconds

1414904400 - 1414907999 ( 2014-11-02 05:00:00 - 2014-11-02 05:59:59 )
test script result: 0
1414908000 - 1414911599 ( 2014-11-02 06:00:00 - 2014-11-02 06:59:59 )
test script result: -3600
1446354000 - 1446357599 ( 2015-11-01 05:00:00 - 2015-11-01 05:59:59 )
test script result: 0
1446357600 - 1446361199 ( 2015-11-01 06:00:00 - 2015-11-01 06:59:59 )
test script result: -3600

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// P1D - adding 1 day. Expected timestamp differential : 86400 seconds

1394265600 - 1394348399  ( 2014-03-08 08:00:00 - 2014-03-09 06:59:59 )
test script result: 82800
1414821600 - 1414907999 ( 2014-11-01 06:00:00 - 2014-11-02 05:59:59 )
test script result: 90000
1425715200 - 1425797999 ( 2015-03-07 08:00:00 - 2015-03-08 06:59:59 )
test script result: 82800
1446271200 - 1446357599 ( 2015-10-31 06:00:00 - 2015-11-01 05:59:59 )
test script result: 90000

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Test script:
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// e.g. adding 1 minute to a DateTime object. The difference should be 60 but it returns -3560.

$ts = 1414907940;
$d = date('Y-m-d  H:i:s', $ts);
$dt = new DateTime($d);
$di = new DateInterval('PT1M');
$dt->add($di);

echo ($dt->getTimestamp() - $ts);  // expecting 60 


Expected result:
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add the correct minute/hour/day to a DateTime object for any DateTime object

Actual result:
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For some time frames, it sometimes adds more, sometimes adds less number of seconds to a Datetime object. (see description)