Description:
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Relative date/time format '+[number]sec', '+[number]min', '+[number]hour' returns incorrect time when the current time is in daylight saving time and the result of calculation is non-daylight saving time.

For example, daylight saving time of this year in the USA was started at 2010-03-14 02:00:00, and finished at 2010-11-07 02:00:00. However, '2010-11-07 01:59:59 +1sec' advance to 3601 sec after.

Test script:
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<?php
date_default_timezone_set('America/Los_Angeles');
echo date("c", strtotime("2010-03-14 01:59:59")) . PHP_EOL;
echo date("c", strtotime("2010-03-14 01:59:59 +1sec")) . PHP_EOL;
echo date("c", strtotime("2010-11-07 01:59:59")) . PHP_EOL;
echo date("c", strtotime("2010-11-07 01:59:59 +1sec")) . PHP_EOL;


Expected result:
----------------
2010-03-14T01:59:59-08:00
2010-03-14T03:00:00-07:00
2010-11-07T01:59:59-07:00
2010-11-07T01:00:00-08:00

Actual result:
--------------
2010-03-14T01:59:59-08:00
2010-03-14T03:00:00-07:00
2010-11-07T01:59:59-07:00
2010-11-07T02:00:00-08:00