Description:
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Using setTimestamp after modifying a DateTime object with "first day of this month" string results in the incorrect date and time.

If "first day of this month" is replaced with something else like "tomorrow" it works fine. For some reason the example below appears to lock in the date component, but not the time.

Test script:
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$time_start = new DateTime('2014-10-02');

echo $time_start->format('r') . "\n";

$time_start->modify('first day of this month');

echo $time_start->format('r') . "\n";

$time_start->setTimestamp(time());

echo $time_start->format('r') . "\n";

Expected result:
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Current date and time

Actual result:
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October 1st