Can't get previous month from DateTime in PHP- Is this a (pretty big) bug?

I need to create functions in PHP that let me step up/down given datetime units. Specifically, I need to be able to move to th开发者_StackOverflowe next/previous month from the current one.

I thought I could do this using DateTime::add/sub(P1M). However, when trying to get the previous month, it messes up if the date value = 31- looks like it's actually trying to count back 30 days instead of decrementing the month value!:

$prevMonth = new DateTime('2010-12-31'); 

Try to decrement the month:

$prevMonth->sub(new DateInterval('P1M')); // = '2010-12-01'
$prevMonth->add(DateInterval::createFromDateString('-1 month')); // = '2010-12-01'
$prevMonth->sub(DateInterval::createFromDateString('+1 month')); // = '2010-12-01'
$prevMonth->add(DateInterval::createFromDateString('previous month')); // = '2010-12-01'

This certainly seems like the wrong behavior. Anyone have any insight? Thanks-

NOTE: PHP version 5.3.3

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(Credit actually belongs to Alex for pointing this out in the comments)

The problem is not a PHP one but a GNU one, as outlined here:

Relative items in date strings

The key here is differentiating between the concept of 'this date last month', which, because months are 'fuzzy units' with different numbers of dates, is impossible to define for a date like Dec 31 (because Nov 31 doesn't exist), and the concept of 'last month, irrespective of date'.

If all we're interested in is the previous month, the only way to gaurantee a proper DateInterval calculation is to reset the date value to the 1st, or some other number that every month will have.

What really strikes me is how undocumented this issue is, in PHP and elsewhere- considering how much date-dependent software it's probably affecting.

Here's a safe way to handle it:

/*
Handles month/year increment calculations in a safe way,
avoiding the pitfall of 'fuzzy' month units.

Returns a DateTime object with incremented month/year values, and a date value == 1.
*/
function incrementDate($startDate, $monthIncrement = 0, $yearIncrement = 0) {

    $startingTimeStamp = $startDate->getTimestamp();
    // Get the month value of the given date:
    $monthString = date('Y-m', $startingTimeStamp);
    // Create a date string corresponding to the 1st of the give month,
    // making it safe for monthly/yearly calculations:
    $safeDateString = "first day of $monthString";
    // Increment date by given month/year increments:
    $incrementedDateString = "$safeDateString $monthIncrement month $yearIncrement year";
    $newTimeStamp = strtotime($incrementedDateString);
    $newDate = DateTime::createFromFormat('U', $newTimeStamp);
    return $newDate;
}

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Easiest way to achieve this in my opinion is using mktime.

Like this:

$date = mktime(0,0,0,date('m')-1,date('d'),date('Y'));
echo date('d-m-Y', $date);

Greetz Michael

p.s mktime documentation can be found here: http://nl2.php.net/mktime

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You could go old school on it and just use the date and strtotime functions.

$date = '2010-12-31';
$monthOnly = date('Y-m', strtotime($date));
$previousMonth = date('Y-m-d', strtotime($monthOnly . ' -1 month'));

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(This maybe should be a comment but it's to long for one)

Here is how it works on windows 7 Apache 2.2.15 with PHP 5.3.3:

<?php $dt = new DateTime('2010-12-31');
$dt->sub(new DateInterval('P1M'));
print $dt->format('Y-m-d').'<br>';
$dt->add(DateInterval::createFromDateString('-1 month'));
print $dt->format('Y-m-d').'<br>';
$dt->sub(DateInterval::createFromDateString('+1 month'));
print $dt->format('Y-m-d').'<br>';
$dt->add(DateInterval::createFromDateString('previous month'));
print $dt->format('Y-m-d').'<br>'; ?>

2010-12-01
2010-11-01
2010-10-01
2010-09-01

So this does seem to confirm it's related to the GNU above.

Note: IMO the code below works as expected.

 $dt->sub(new DateInterval('P1M'));

Current month: 12
Last month: 11
Number of Days in 12th month: 31
Number of Days in 11th month: 30
Dec 31st - 31 days = Nov 31st
Nov 31st = Nov 1 + 31 Days = 1st of Dec (30+1)