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phpdatetimedatediffdateinterval

DateTime::diff weired result

I use DateTime::diff to compare two DateTime, but the result is very weird as if abs($date1 - $date2) != abs($date2 - $date1).

$date1 = new DateTime("1980-11-21 00:00:00");
$date2 = new DateTime("1981-11-20 00:00:00");
var_dump($date1->diff($date2,true)->days);
var_dump($date2->diff($date1,true)->days);
var_dump($date1->diff($date2,true)->format("%Y-%m-%d %H:%i:%s"));
var_dump($date2->diff($date1,true)->format("%Y-%m-%d %H:%i:%s"));

Which returns:

int(364)
int(364)
string(15) "00-11-30 00:0:0"
string(15) "00-11-29 00:0:0"

Demo

Solution

  • The behavior is correct. Both date intervals represent same number of days (364). The month and date part represent the period that needs to be added to/subtracted from first date to reach second date:

    1981-11-20 minus 1980-11-21 represents an interval of 11 month 30 day
1980-11-21 + 11 month = 1981-10-21
1981-10-21 + 30 day   = 1981-10-21

1980-11-21 minus 1981-11-20 represents an interval of 11 month 29 day
1981-11-20 - 11 month = 1980-12-20
1980-12-20 - 29 day   = 1980-11-21

    A simpler example would be Oct 21 ... Dec 20:

    • You need to add 1 month 29 days to Oct 21 to reach Dec 20 (note that month before Dec has 30 days)
    • But ... you need to subtract 1 month 30 days from Dec 20 to reach Oct 21. Oct itself has 31 days which explains the difference.