Re: DateInterval bug

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... cheers, Derick -- http://derickrethans.nl | http://xdebug.org Like Xdebug? Consider a donation: http://xdebug.org/donate.php twitter: @derickr and @xdebug Posted with an email client that doesn't mangle email: alpine