Zend_Date offers different methods to help working with dates. One of them is the "subMonth" method.

I have been using this method to work trough dates and performing DB queries. For example:

$date = new Zend_Date('2012-03-01 00:00:00');
$lastMonth = $date->subMonth(1); //Expected value is '2012-02-01 00:00:00'

This works for every case, except when the date is 'YEAR-02-01 00:00:00'. In the case the date is February 1st, the date returned it January 1st of the previous year.

$date = new Zend_Date('2012-02-01 00:00:00');
$lastMonth = $date->subMonth(1); //Resulted value is '2011-01-01 00:00:00'

Interesting fact is if you continue to iterate trough this, the date gets back to normal, but it will fail in every February 1st.

$date = new Zend_Date('2012-02-01 00:00:00');
while ($months++ < 24) {
    echo $date->toString('YYYY-MM-dd') . '
';
    $date->subMonth(1);
}

This will output:

2012-02-01
2011-01-01 << Wrong year
2011-12-01
2011-11-01
...
2011-03-01
2011-02-01
2010-01-01 << Wrong year
2010-12-01
2010-11-01
...

I noticed others strange behaviors, but I was unable to replicate as this.

The file version is described as: * @version $Id: Date.php 22713 2010-07-29 11:41:56Z thomas $ It's the version included in the Magento library.

I have upgraded this component to the 1.11.10, and the problem seems to persists. * @version $Id: Date.php 24108 2011-06-04 00:09:27Z freak $