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Date: Sun, 12 Apr 2015 20:57:55 +0200
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Subject: DateInterval bug
From: peter.e.lind@gmail.com (Peter Lind)

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Hi,

I wanted to get into PHP code development so I grabbed a random bug from
bugs.php.net. Which turned out to be https://bugs.php.net/bug.php?id=69378

The problem the bug report describes is that creating a diff between two
dates and then subtracting the diff from the later date does not give you
the former date. Or, as the bug report state, this should hold:

B - (B - A) == A

But it doesn't, because of the way DateInterval and DateTime interact. A
DateInterval is broken up into years, months, days, hours, minutes and
seconds - which can be added or subtracted from a date. However, months are
not fit size, so the order in which things are added or subtracted matters.
In the bug report, the problem arises because months are subtracted before
days - and there's a huge difference between subtracting 17 days from 2.
April and from 2. March.

In itself, this isn't a big problem - but apparently this behaviour is how
the system is supposed to work. In the tests for the date extension, I
found this test for DateTime::add

echo "test_years_positive__6_shy_1_day: ";
examine_diff('2007-02-06', '2000-02-07', 'P+6Y11M30DT0H0M0S', 2556);

echo "test_years_negative__6_shy_1_day: ";
examine_diff('2000-02-07', '2007-02-06', 'P-6Y11M28DT0H0M0S', 2556);

The third argument in the examine_diff calls is used in the constructor
call to DateInterval. The difference is whether the interval will be
positive or negative. Note the difference of two days - if you add a
positive interval, then 7 years minus 1 day is 6 years, 11 months and 30
days. If you add a negative interval, then 7 years minus 1 day is 6 years,
11 months and 28 days.

Is there a good explanation for this behaviour (which applies both to
DateTime::add and DateTime::sub)? I've tried searching the internals list
but couldn't see any discussion of it. It seems like a bug that never got
fixed to the point where there are tests to make sure things are still
calculated wrong.

Regards
Peter

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