如何检查DST（夏令时）是否有效，以及偏移是否有效？ [英] How to check if DST (Daylight Saving Time) is in effect, and if so, the offset?

问题描述

这是我的JS代码所需要的：

This is a bit of my JS code for which this is needed:

var secDiff = Math.abs(Math.round((utc_date-this.premiere_date)/1000));
this.years = this.calculateUnit(secDiff,(86400*365));
this.days = this.calculateUnit(secDiff-(this.years*(86400*365)),86400);
this.hours = this.calculateUnit((secDiff-(this.years*(86400*365))-(this.days*86400)),3600);
this.minutes = this.calculateUnit((secDiff-(this.years*(86400*365))-(this.days*86400)-(this.hours*3600)),60);
this.seconds = this.calculateUnit((secDiff-(this.years*(86400*365))-(this.days*86400)-(this.hours*3600)-(this.minutes*60)),1);

我想获取 ago中的日期时间，但是如果使用了DST，则日期下班1小时。我不知道如何检查DST是否有效。

I want to get the datetime in "ago", but if the DST is in use then the dates are off by 1 hour. I don't know how to check if the DST is in effect or not.

我怎么知道夏令时的开始和结束时间？

How can I know when the daylight saving starts and ends?

推荐答案

此代码使用以下事实： getTimezoneOffset 返回一个更大值在标准时间与夏令时（DST）之间。因此，它确定标准时间期间的预期输出，并比较给定日期的输出是否相同（标准）或更少（ DST）。

This code uses the fact that getTimezoneOffset returns a greater value during Standard Time versus Daylight Saving Time (DST). Thus it determines the expected output during Standard Time, and it compares whether the output of the given date the same (Standard) or less (DST).

请注意， getTimezoneOffset 会返回 west ，通常表示为负小时（因为它们位于UTC之后）。例如，洛杉矶是 UTC-8h 标准， UTC-7h DST。 getTimezoneOffset 返回12月（冬季，标准时间）的 480 （正480分钟），而不是 -480 。它会返回东半球的负编号（例如，冬季在悉尼为 -600 ，尽管这是超前的（ UTC + 10h ）。

Note that getTimezoneOffset returns positive numbers of minutes for zones west of UTC, which are usually stated as negative hours (since they're "behind" UTC). For example, Los Angeles is UTC–8h Standard, UTC-7h DST. getTimezoneOffset returns 480 (positive 480 minutes) in December (winter, Standard Time), rather than -480. It returns negative numbers for the Eastern Hemisphere (such -600 for Sydney in winter, despite this being "ahead" (UTC+10h).

Date.prototype.stdTimezoneOffset = function () {
    var jan = new Date(this.getFullYear(), 0, 1);
    var jul = new Date(this.getFullYear(), 6, 1);
    return Math.max(jan.getTimezoneOffset(), jul.getTimezoneOffset());
}

Date.prototype.isDstObserved = function () {
    return this.getTimezoneOffset() < this.stdTimezoneOffset();
}

var today = new Date();
if (today.isDstObserved()) { 
    alert ("Daylight saving time!");
}

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