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<?php $ss = 'first day of last month midnight'; $es = 'first day of this month midnight - 1 second'; $s = new DateTime($ss); $e = new DateTime($es); $d= $e->diff($s); var_dump($d->days); // 0 ... but should be 30 $s = (new DateTime(null))->setTimestamp(strtotime($ss)); // verbose setup method $e = (new DateTime(null))->setTimestamp(strtotime($es)); // verbose setup method $d = $e->diff($s); var_dump($d->days); // 30 ... and should be 30 /* Next we will try mix/match the code to see what happens, surprisingly it seems that the end date ($e) is the important one, if it uses the verbose method it returns the correct values. */ $s = (new DateTime(null))->setTimestamp(strtotime($ss)); // verbose setup method $e = new DateTime($es); $d= $e->diff($s); var_dump($d->days); // 0 ... but should be 30 $s = new DateTime($ss); $e = (new DateTime(null))->setTimestamp(strtotime($es)); // verbose setup method $d= $e->diff($s); var_dump($d->days); // 30 ... and should be 30 /* This test just proves that the $e date is important BUT NOT because it's the one we call the diff() method on, that's just coincidental that seems to imply that the "- 1 second" in the date string is the problem. */ $s = new DateTime($ss); $e = (new DateTime(null))->setTimestamp(strtotime($es)); // verbose setup method $d= $s->diff($e); var_dump($d->days); // 30 ... and should be 30 /* [Workaround] This final test seems to prove that the input string is important and that the "- 1 secord" has a negative knock-on effect on the results of the diff. By modifying the datetime with ->modify everything works as expected ... it just means you have to be careful of how we work with DateTimes . */ $s = new DateTime($ss); $e = new DateTime('first day of this month midnight'); $e->modify('- 1 second'); var_dump($e->diff($s)->days); // 30 ... and should be 30 echo "\n\n Same stuff with DateTimeImmutable\n"; $ss = 'first day of last month midnight'; $es = 'first day of this month midnight - 1 second'; $s = new DateTimeImmutable($ss); $e = new DateTimeImmutable($es); $d= $e->diff($s); var_dump($d->days); // 0 ... but should be 30 $s = (new DateTimeImmutable(null))->setTimestamp(strtotime($ss)); // verbose setup method $e = (new DateTimeImmutable(null))->setTimestamp(strtotime($es)); // verbose setup method $d = $e->diff($s); var_dump($d->days); // 30 ... and should be 30 /* Next we will try mix/match the code to see what happens, surprisingly it seems that the end date ($e) is the important one, if it uses the verbose method it returns the correct values. */ $s = (new DateTimeImmutable(null))->setTimestamp(strtotime($ss)); // verbose setup method $e = new DateTimeImmutable($es); $d= $e->diff($s); var_dump($d->days); // 0 ... but should be 30 $s = new DateTimeImmutable($ss); $e = (new DateTimeImmutable(null))->setTimestamp(strtotime($es)); // verbose setup method $d= $e->diff($s); var_dump($d->days); // 30 ... and should be 30 /* This test just proves that the $e date is important BUT NOT because it's the one we call the diff() method on, that's just coincidental that seems to imply that the "- 1 second" in the date string is the problem. */ $s = new DateTimeImmutable($ss); $e = (new DateTimeImmutable(null))->setTimestamp(strtotime($es)); // verbose setup method $d= $s->diff($e); var_dump($d->days); // 30 ... and should be 30 /* [Workaround] This final test seems to prove that the input string is important and that the "- 1 secord" has a negative knock-on effect on the results of the diff. By modifying the datetime with ->modify everything works as expected ... it just means you have to be careful of how we work with DateTimes . */ $s = new DateTimeImmutable($ss); $e = new DateTimeImmutable('first day of this month midnight'); $e->modify('- 1 second'); var_dump($e->diff($s)->days); // 30 ... and should be 30