日期构造函数：数字参数与字符串参数在某些情况下给出不同的日期 [英] Date constructor: numeric arguments vs. string argument giving different dates in some cases

问题描述

首先，我认为时区可能与这有关。我在EST / EDT。

现在，我们创建两个日期：

  // 12月5日
 
 dateFromNumbers = new Date（2020，11，5）; 
 dateFromString = new Date（2020-12-5）; 
  

看起来这些日期应该有相同的时间戳，并且它们：

  + dateFromNumbers == + dateFromString; // true 
  

...至少在这种情况下。但在某些情况下，它们不会：

  // 12月15日
 
 dateFromNumbers = （2020，11，15）; 
 dateFromString = new Date（2020-12-15）; 
 
 + dateFromNumbers == + dateFromString; // false 
  

这是怎么回事？

  dateFromNumbers; // Tue Dec 15 2020 00:00:00 GMT-0500（EST）
 dateFromString; // Mon Dec 14 2020 19:00:00 GMT-0500（EST）
  

看起来像 dateFromString 比 dateFromNumbers 早5小时（EST是GMT - 5，我确定它是相关的）。

这似乎影响了10月至12月的月底。这是一个小提琴，它使得很容易看到哪些天不同（除非你是红绿色的颜色，在这种情况下可能很难看到，我的道歉）。

http://jsfiddle.net/9gBfX/

有什么优点？ / p>

---

注意：

• 可以将系统时区设置为EST / EDT，以查看jsfiddle示例，因为我看到它。


• 日期的月份数基于零; 不是打字错误。


• 这个问题出现在我检查的每一年。

解决方案

查看 V8的源代码：

  //规格：
 //使用Safari接受ES5 ISO 8601日期时间字符串或旧日期兼容
 //。 
< ...> 
 //匹配这两种格式的字符串（例如1970-01-01）将是
 //解析为ES5日期时间字符串 - 这意味着它将默认
 //到UTC时区。如果遵循ES5 
 //规范，这是不可避免的。 
 

读取周围的代码，看起来是一个日期 - 时间字符串，包含月份和日期2个符号被认为是有效的ES5日期时间字符串。在ES5解析器中，在解析日期和时间之后，有一个注释：

  //成功解析ES5日期时间串。默认为UTC，如果没有给定TZ。 
 

在YYYY-MM-DD的情况下，一旦代码到达远处，ES5解析器已成功解析整个字符串，因此它在旧解析器获得本地化时区的机会之前返回。否则（月/日为一个符号长），它被视为遗留日期时间，旧的解析器将处理它并将其本地化。

First of all, I think timezone probably has something to do with this. I'm in EST/EDT. Also, I'm testing this on chromium 17 / linux.

Now, let's say I create two dates like this:

// December 5

dateFromNumbers = new Date(2020, 11, 5);
dateFromString = new Date("2020-12-5");

It seems these dates should have identical timestamps, and they do:

+dateFromNumbers == +dateFromString; // true

...at least in this case. But in some cases, they don't:

// December 15

dateFromNumbers = new Date(2020, 11, 15);
dateFromString = new Date("2020-12-15");

+dateFromNumbers == +dateFromString; // false

What's going on here?

dateFromNumbers; // Tue Dec 15 2020 00:00:00 GMT-0500 (EST)
dateFromString;  // Mon Dec 14 2020 19:00:00 GMT-0500 (EST)

Looks like dateFromString is 5 hours earlier than dateFromNumbers in this case (EST is GMT - 5, I'm sure it's related somehow).

It seems to affect the ends of months October through December. Here's a fiddle that makes it easy to see which days differ (unless you are red-green colorblind, in that case it may be difficult to see, my apologies).

http://jsfiddle.net/9gBfX/

What gives?

---

Notes:

• You can set your system timezone to EST/EDT to see the jsfiddle example as I'm seeing it.
• Date's month numbers are zero-based; the 11 is not a typo.
• This issue appears in every year that I checked.

解决方案

After looking in V8's source code:

// Specification:
// Accept ES5 ISO 8601 date-time-strings or legacy dates compatible
// with Safari.
<...>
//  A string that matches both formats (e.g. 1970-01-01) will be
//  parsed as an ES5 date-time string - which means it will default
//  to UTC time-zone. That's unavoidable if following the ES5
//  specification.

Reading the surrounding code, it seems that a date-time string with both month and day 2 symbols long is considered a valid ES5 date-time string. Further down in the ES5 parser, after it parses the dates and times, there's a comment:

// Successfully parsed ES5 Date Time String. Default to UTC if no TZ given.

In case of "YYYY-MM-DD", once the code gets that far, the ES5 parser has successfully parsed the entire string, so it returns before the legacy parser gets a chance to localize the timezone. Otherwise (month/day are one symbol long) it's treated as a "legacy" datetime and the legacy parser gets to deal with it and localize it.

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