Bug #81527 [Com]: DateTime::diff returns different results when called on both objects.

From: Date: Mon, 18 Oct 2021 08:07:53 +0000
Subject: Bug #81527 [Com]: DateTime::diff returns different results when called on both objects.
References: 1  Groups: php.bugs 
Request: Send a blank email to php-bugs+get-237251@lists.php.net to get a copy of this message 
Edit report at https://bugs.php.net/bug.php?id=81527&edit=1

 ID:                 81527
 Comment by:         victor dot todoran at yahoo dot om
 Reported by:        victor dot todoran at yahoo dot com
 Summary:            DateTime::diff returns different results when called
                     on both objects.
 Status:             Not a bug
 Type:               Bug
 Package:            Date/time related
 Operating System:   Ubuntu 20.04
 PHP Version:        7.4.24
 Block user comment: N
 Private report:     N

 New Comment:

Thanks for taking the time to respond and give an example.


Previous Comments:
------------------------------------------------------------------------
[2021-10-15 14:46:05] requinix@php.net

The number of days *is* the same.

echo $diff1->days; // 730
echo $diff2->days; // 730

What's different is the *representation* of those days.

2015-10-25
+ 1 year = 2016-10-25
+ 11 months = 2017-09-25
+ 29 days = 2017-10-24

2017-10-24
- 1 year = 2016-10-24
- 11 months = 2015-11-24
- 30 days = 2015-10-25

Dates are hard™.

Advice: humans typically measure differences by counting from the lower number to the higher number
so you'll probably want to use $diff1.

------------------------------------------------------------------------
[2021-10-15 11:39:04] victor dot todoran at yahoo dot com

Description:
------------
DateTime::diff returns different results when called on both objects.

Test script:
---------------
<?php

$date1 = new \DateTime('2015-10-25', new \DateTimeZone('UTC'));
$date2 = new \DateTime('2017-10-24', new \DateTimeZone('UTC'));

$diff1 = $date1->diff($date2);
$diff2 = $date2->diff($date1);

echo $diff1->y . ' years ' . $diff1->m . ' months ' . $diff1->d .
' days '; // outputs 1 years 11 months 29 days 
echo PHP_EOL;
echo $diff2->y . ' years ' . $diff2->m . ' months ' . $diff2->d .
' days '; // outputs 1 years 11 months 30 days 

Expected result:
----------------
1 years 11 months 29 days 
1 years 11 months 29 days

or 

1 years 11 months 30 days 
1 years 11 months 30 days

I would expect the number of days to be identical.


Actual result:
--------------
The number of days differs 

1 years 11 months 29 days 
1 years 11 months 30 days


------------------------------------------------------------------------



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