如何检查DST（夏令时）是否有效以及它是否是偏移量？ [英] How to check if the DST (Daylight Saving Time) is in effect and if it is what's the offset?

问题描述

这是我需要的一些JS代码：

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);

我想在之前获得日期时间，但如果正在使用DST，那么日期将会关闭1小时。我不知道如何检查DST是否正在使用。

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 use 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 getTimezoneOffset 返回更大值的事实在标准时间与夏令时（DST）期间。因此，它确定标准时间内的预期输出，并比较给定日期的输出是否相同（标准）或更低（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).

注意 getTimezoneOffset 返回区域 west <的正分钟数UTC的/ em>，通常表示为负小时（因为它们落后于UTC）。例如，洛杉矶 UTC-8h 标准， UTC-7h DST。 getTimezoneOffset 12月（冬季，标准时间）返回 480 （正480分钟），而不是 -480 。它返回东半球的负数字（冬季悉尼的 -600 ，尽管这是领先（ 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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