Adding to a time provides an inconsistent result

[gregrobson]

Firstly I love the library (especially as part of Laravel) and this is a PHP issue more than a Carbon issue.

When adding/subtracting times for a value against a Timezone other than UTC, the add/subtract features do not take daylight saving into consideration

<?php
// Clocks go forward on this day: 00:59:59 > 02:00:00
$a = new Carbon('2014-03-30 00:00:00', 'Europe/London'); 
$b = $a->copy();
$b->addHours(24);
echo $b; // Produces 2014-03-31 00:00:00, expected 2014-03-31 01:00:00
echo $a->diffInHours($b); // Produces 23, expected 24.

It seems that PHP's modify() function ignores daylight saving values entirely. The documentation often mentions that comparisons are always done in UTC, so I would expect the additions to add UTC units of time as well.

This also leads the possibility of time loops!
http://uk3.php.net/manual/en/datetime.modify.php#113549

[gregrobson]

My fix for this

<?php
$a = new Carbon('2014-03-30 00:00:00', 'Europe/London'); 
$b = $a->copy();

$b->setTimezone('UTC'); // Change to UTC for correct addition
$b->addHours(24);
$b->setTimezone('Europe/London'); // Change back for correct display value.

echo $b; // Produces 2014-03-31 01:00:00
echo $a->diffInHours($b); // Produces 24

[kylekatarnls]

It seems to be an error due to the PHP DateTime object, so I propose to put you hack in a overrided modify method.
Suggested pull request