Why does PHP date('m', strtotime('-1 months')) not work correctly for today? 07/31

Question

I have a script that gets the current and last month in PHP like so:

$currentMonth = date('m');
//Expected:07
//Result:07
$lastMonth = date('m', strtotime('-1 months'));
//Expected:06
//Result:07

Today happens to be the 31 or end of the month of July. Is this result to be expected from PHP?

When using -31 days the result is as expected:

$lastMonth = date('m', strtotime('-31 days'));
//Expected:06
//Result:06

Answer 1

Here's a cleaner test case that doesn't expire:

<?php
$origin = mktime(18, 0, 0, 7, 31, 2015);
var_dump( date('r', $origin), date('r', strtotime('-1 months', $origin)) );

string(31) "Fri, 31 Jul 2015 18:00:00 +0200" 
string(31) "Wed, 01 Jul 2015 18:00:00 +0200"

I'm pretty sure it's a documentation issue, because the manual clearly states this (emphasis mine):

Relative month values are calculated based on the length of months that they pass through. An example would be "+2 month 2011-11-30", which would produce "2012-01-30". This is due to November being 30 days in length, and December being 31 days in length, producing a total of 61 days.

... and it's wrong.

PHP bug tracker has tons of dupes about this. They're all closed as not a bug. Here's a relevant comment from 2009 that explains it:

I agree that this is an annoying behaviour.

Also, the implementation is problematic. Basically if you use '+1 month' it takes the month number, adds 1 and parses result as a new date.

If you use '+1 month' on the first of the month, it sets the date to the next first of the month.

This behaviour gives the impression, that php considers the length of a month, which is not true.

But if you use '+1 month' on the last day of a month, the result is unexpected as 2009-05-31 becomes 2009-06-31 which is an invalid date and then interpreted as 2009-07-01.

This should at least be mentioned in the documentation.

Answer 2

You can do this way

$d = new DateTime(); 
$currentMonth = $d->format('m');
//Expected:07
//Result:07
print $currentMonth;

$d->modify('first day of previous month');
print "<br/>";
$lastMonth = $d->format('m'); 
//Expected:06
//Result:06
print $lastMonth;

DEMO: http://codepad.viper-7.com/kokWi8