Hi All. Am 14.04.15 um 08:38 schrieb Peter Lind: > On 13 April 2015 at 22:20, Derick Rethans <der...@php.net> wrote: > >> On Sun, 12 Apr 2015, Peter Lind wrote: >> >>> 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. >> >> Why is it a bug? With DateTime math, reversing an operation isn't >> necessarily going to work... >> >> > Math that isn't consistent is problematic, in my book. Or, to put it > another way: if someone told me, that there is 6 years, 11 months and 30 > days between 7th Feb 2000 and 6th Feb 2007, I would agree. If the same > person then told me that the interval *is also* 6 years, 11 months and 28 > days *and that both intervals are correct* - I would question the sanity of > that person. I would go so far as to say: don't ever manage my calendar. > > The bug as I see it is that two classes/behaviours have been packed into > one - there should be DateIntervalRepresentation and DateInterval. The > first shows you how many years, months, days, etc there are between two > dates. The second is the actual interval between two dates. The first is > context dependent - it needs anchoring in at least one date. The second is > context independent. > > Either that or do away with math you can't rely on. I think what we should > aim for in programming languages is the opposite of "isn't necessarily > going to work..." > > Regards > Peter >
As far as I can see in http://3v4l.org/mUeRk in PHP 5.3.2 the behaviour was still as you'd expect it, in PHP5.3.3 it was as described in the ticket. So it seems that there has been a change introduced with PHP5.3.3 that "broke" the "expected" behaviour. Cheers Andreas -- ,,, (o o) +---------------------------------------------------------ooO-(_)-Ooo-+ | Andreas Heigl | | mailto:andr...@heigl.org N 50°22'59.5" E 08°23'58" | | http://andreas.heigl.org http://hei.gl/wiFKy7 | +---------------------------------------------------------------------+ | http://hei.gl/root-ca | +---------------------------------------------------------------------+
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