Question

Being that the current month/year is January 2012, why does the following code return December 2011 and not November 2011?

echo date("F Y", strtotime("-2 months"));

This is on PHP 5.3.0 if it makes a difference.

Answer 1

To get what you are looking for you can use this rather verbose version instead:

echo date("F Y", strtotime("first day of this month - 2 months"));

The problem with your original version is described in detail here: http://derickrethans.nl/obtaining-the-next-month-in-php.html. Quoted below:

Over and over again PHP users complain that next month in PHP’s
date-string parser doesn’t go to the next month, but instead skips to
the one after next month; like in the following example:

<?php
$d = new DateTime( '2010-01-31' );
$d->modify( 'next month' );
echo $d->format( 'F' ), "\n";
?>

The output of the little script will be March. March obviously doesn’t
follow January as February is in between. However, the current
behavior is correct. The following happens internally:

next month increases the month number (originally 1) by one. This
makes the date 2010-02-31. The second month (February) only has 28
days in 2010, so PHP auto-corrects this by just continuing to count
days from February 1st. You then end up at March 3rd. The formatting
strips off the year and day, resulting in the output March. This can
easily be seen when echoing the date with a full date format, which
will output March 3rd, 2010:

This is for adding months, but the same applies in reverse when subtracting months; there was no November 31st, so the strtotime method “corrects” it into December 1st.