PHP DateTime时差仅在UTC时区中正确

谁能解释一下我的行为:

$end = new DateTime("2015-03-01", new DateTimeZone("Europe/Berlin"));
$start = new DateTime("2015-02-01", new DateTimeZone("Europe/Berlin"));
$diff = $start->diff($end);
//result: DateInterval Object ( [y] => 0 [m] => 0 [d] => 28 [h] => 0 [i] => 0 [s] => 0 [invert] => 0 [days] => 28 ) 

如果我不使用时区或欧洲/柏林，我会得到错误的时差。28天确实是正确的，但在这种情况下是一个月。

$end = new DateTime("2015-03-01", new DateTimeZone("UTC"));
$start = new DateTime("2015-02-01", new DateTimeZone("UTC"));
$diff = $start->diff($end);
//result: DateInterval Object ( [y] => 0 [m] => 1 [d] => 0 [h] => 0 [i] =>; 0 [s] => 0 [invert] => 0 [days] => 28 )

与UTC，它工作

$end = new DateTime("2015-04-01", new DateTimeZone("Europe/Berlin"));
$start = new DateTime("2015-03-01", new DateTimeZone("Europe/Berlin"));
$diff = $start->diff($end);
//result: DateInterval Object ( [y] => 0 [m] => 1 [d] => 3 [h] => 0 [i] => 0 [s] => 0 [invert] => 0 [days] => 31 ) 

1个月零3天?二月，你在这里闹鬼吗?

$end = new DateTime("2015-04-01", new DateTimeZone("UTC"));
$start = new DateTime("2015-03-01", new DateTimeZone("UTC"));
$diff = $start->diff($end);
//result: DateInterval Object ( [y] => 0 [m] => 1 [d] => 0 [h] => 0 [i] =>; 0 [s] => 0 [invert] => 0 [days] => 31 )

31天= 1个月，UTC正确

$end = new DateTime("2015-05-01", new DateTimeZone("Europe/Berlin"));
$start = new DateTime("2015-04-01", new DateTimeZone("Europe/Berlin"));
$diff = $start->diff($end);
//result: DateInterval Object ( [y] => 0 [m] => 0 [d] => 30 [h] => 0 [i] => 0 [s] => 0 [invert] => 0 [days] => 30 ) 

欧洲是30天而不是一个月?

$end = new DateTime("2015-05-01", new DateTimeZone("UTC"));
$start = new DateTime("2015-04-01", new DateTimeZone("UTC"));
$diff = $start->diff($end);
//result: DateInterval Object ( [y] => 0 [m] => 1 [d] => 0 [h] => 0 [i] =>; 0 [s] => 0 [invert] => 0 [days] => 30 )

UTC正确

$end = new DateTime("2015-06-01", new DateTimeZone("Europe/Berlin"));
$start = new DateTime("2015-05-01", new DateTimeZone("Europe/Berlin"));
$diff = $start->diff($end);
//result: DateInterval Object ( [y] => 0 [m] => 1 [d] => 1 [h] => 0 [i] => 0 [s] => 0 [invert] => 0 [days] => 31 ) 

又是31天，正确的是1个月，但为什么是+1天?

$end = new DateTime("2015-06-01", new DateTimeZone("UTC"));
$start = new DateTime("2015-05-01", new DateTimeZone("UTC"));
$diff = $start->diff($end);
//result: DateInterval Object ( [y] => 0 [m] => 1 [d] => 0 [h] => 0 [i] =>; 0 [s] => 0 [invert] => 0 [days] => 31 )

UTC仍然正确

我不明白。谢谢你的帮助。