如何確定夏令時 (DST) 是否生效？

Determining DST and Its Offset

To check if Daylight Saving Time (DST) is in effect, you can utilize the getTimezoneOffset property, which returns a time difference in minutes from UTC. The key difference lies in the convention:

• Zones west of UTC (usually "behind") display positive offset values. For instance, Los Angeles would show 480 minutes (8 hours positive) during Standard Time and 420 minutes (7 hours positive) during DST.
• Zones east of UTC display negative offset values.

To determine the DST status, compare the current offset with the standard offset:

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();
};

By calling isDstObserved() on a date object, you can verify if DST is active.

Example:

var today = new Date();
if (today.isDstObserved()) {
    console.log("Daylight saving time is in effect!");
}

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