Bug #80696 [Nab]: Subtracting an interval of 1 month 1 day subtracts one more day

Edit report at https://bugs.php.net/bug.php?id=80696&edit=1 ID: 80696 Updated by: requinix@php.net Reported by: jurchiks101 at gmail dot com Summary: Subtracting an interval of 1 month 1 day subtracts one more day Status: Not a bug Type: Bug Package: Date/time related Operating System: Windows 10 PHP Version: 7.4.14 Block user comment: N Private report: N New Comment: What part, my brief explanation or PHP's behavior? I'll assume the former. That's good because it gives me a chance to mention that I got it backwards. 2021-01-01 minus P1M1D is 2020-11-30. Agreed on that? 2020-11-30 plus P1M = 2020-12-30 2020-12-30 plus P1D = 2021-12-31 Compare with 2020-11-30 plus P1D = 2020-12-01 2020-12-01 plus P1M = 2021-01-01 Same starting date, same ending date, different intervals depending how you count it. Previous Comments: ------------------------------------------------------------------------ [2021-02-02 00:58:48] jurchiks101 at gmail dot com Well that makes no sense. Great! ------------------------------------------------------------------------ [2021-02-02 00:53:33] requinix@php.net You calculate the diff by counting days before months. PHP calculates the diff by counting months before days. Dates are hard. ------------------------------------------------------------------------ [2021-02-01 23:33:09] jurchiks101 at gmail dot com Description: ------------ When subtracting an interval that contains months and days, the number of days subtracted is sometimes increased by an inconsistent amount. Here's the 3v4l: https://3v4l.org/uIWq4 Test script: --------------- $firstDayOfYear = new DateTimeImmutable('2021-01-01 00:00:00.000000'); for ($i = 1; $i <= 12; $i++) { $intervalOneEach = new DateInterval('P' . $i . 'M1D'); echo $firstDayOfYear ->sub($intervalOneEach) ->diff($firstDayOfYear) ->format('%y years %m months %d days %h hours %i minutes %s seconds'), PHP_EOL; } Expected result: ---------------- 0 years 1 months 1 days 0 hours 0 minutes 0 seconds 0 years 2 months 1 days 0 hours 0 minutes 0 seconds 0 years 3 months 1 days 0 hours 0 minutes 0 seconds 0 years 4 months 1 days 0 hours 0 minutes 0 seconds 0 years 5 months 1 days 0 hours 0 minutes 0 seconds 0 years 6 months 1 days 0 hours 0 minutes 0 seconds 0 years 7 months 1 days 0 hours 0 minutes 0 seconds 0 years 8 months 1 days 0 hours 0 minutes 0 seconds 0 years 9 months 1 days 0 hours 0 minutes 0 seconds 0 years 10 months 1 days 0 hours 0 minutes 0 seconds 0 years 11 months 1 days 0 hours 0 minutes 0 seconds 1 years 0 months 1 days 0 hours 0 minutes 0 seconds Actual result: -------------- 0 years 1 months 2 days 0 hours 0 minutes 0 seconds 0 years 2 months 1 days 0 hours 0 minutes 0 seconds 0 years 3 months 2 days 0 hours 0 minutes 0 seconds 0 years 4 months 1 days 0 hours 0 minutes 0 seconds 0 years 5 months 1 days 0 hours 0 minutes 0 seconds 0 years 6 months 2 days 0 hours 0 minutes 0 seconds 0 years 7 months 1 days 0 hours 0 minutes 0 seconds 0 years 8 months 2 days 0 hours 0 minutes 0 seconds 0 years 9 months 1 days 0 hours 0 minutes 0 seconds 0 years 10 months 3 days 0 hours 0 minutes 0 seconds 0 years 11 months 1 days 0 hours 0 minutes 0 seconds 1 years 0 months 1 days 0 hours 0 minutes 0 seconds ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=80696&edit=1