The functionality of new Date() is inconsistent when encountering a daylight savings time switch

In my current situation, I am facing an issue when passing "year, month, date, time" to the Date() function in order to retrieve the datetime type.

  1. Utilizing Google Chrome as the browser.
  2. The Windows system is set to the EST timezone (-05:00).
  3. Daylight Saving Time (DST) begins on March 8th, 2020 at 2AM (EDT -04:00)

When entering the date and time as Date(2020,02,08,01), the output is displayed as "Sun Mar 08 2020 01:00:00 GMT-0500 (Eastern Standard Time)".

However, if the date and time specified includes the exact moment of the DST switch at 2AM, the result appears as "Sun Mar 08 2020 03:00:00 GMT-0400 (Eastern Daylight Time)".

This discrepancy results in the time being represented as "03" instead of "02".

I am seeking clarification on why the Date function does not behave as anticipated for this specific time period, as well as any solutions to resolve this issue.

javascriptangulartypescript

Answer №1

We are observing DST on March 29th, and everything is functioning as expected in the GMT-1/GMT-2 time zone.

console.log(new Date(2020,2,29,0,0,0,0)); // The output in CEST is "2020-03-28T23:00:00.000Z"
console.log(new Date(2020,2,29,1,0,0,0))  // The output in CEST is "2020-03-29T00:00:00.000Z"
console.log(new Date(2020,2,29,2,0,0,0))  // The output in CEST is "2020-03-29T01:00:00.000Z"
console.log(new Date(2020,2,29,3,0,0,0))  // The output in CEDT is "2020-03-29T01:00:00.000Z" !!!
console.log(new Date(2020,2,29,4,0,0,0))  // The output in CEDT is "2020-03-29T02:00:00.000Z"

You might consider using UTC instead.

console.log(new Date(Date.UTC(2020,2,29,0,0,0,0))); // The output in CET is "2020-03-29T00:00:00.000Z"
console.log(new Date(Date.UTC(2020,2,29,1,0,0,0))); // The output in CET is "2020-03-29T01:00:00.000Z"
console.log(new Date(Date.UTC(2020,2,29,2,0,0,0))); // The output in CET is "2020-03-29T02:00:00.000Z"
console.log(new Date(Date.UTC(2020,2,29,3,0,0,0))); // The output in CET is "2020-03-29T03:00:00.000Z" 
console.log(new Date(Date.UTC(2020,2,29,4,0,0,0))); // The output in CET is "2020-03-29T04:00:00.000Z"

Alternatively, you could use UTC on the 8th:

console.log(new Date(Date.UTC(2020,2,8,0,0,0,0))); // The output in CET is "2020-03-08T00:00:00.000Z"
console.log(new Date(Date.UTC(2020,2,8,1,0,0,0))); // The output in CET is "2020-03-08T01:00:00.000Z"
console.log(new Date(Date.UTC(2020,2,8,2,0,0,0))); // The output in CET is "2020-03-08T02:00:00.000Z"
console.log(new Date(Date.UTC(2020,2,8,3,0,0,0))); // The output in CET is "2020-03-08T03:00:00.000Z" 
console.log(new Date(Date.UTC(2020,2,8,4,0,0,0))); // The output in CET is "2020-03-08T04:00:00.000Z"

Answer №2

The outcome is accurate.

When using Date(2020,02,08,01,59,00), it results in Sun Mar 08 2020 01:59:00 GMT-0500
Date(2020,02,08,02,00,00) will give you Sun Mar 08 2020 03:00:00 GMT-0400

During Spring, clocks move forward by an hour. You'll notice the GMT offset decreasing by one hour due to daylight savings time on the East Coast.

There is no 2AM (GMT-05) on March 8 in the Eastern timezone. After 01:59:59 GMT-05:00, the clock jumps directly to 03:00:00 GMT -04:00. This jump reflects the change in GMT, which acts as an offset.

Internally, Javascript stores the exact time in ticks from 1/1/1970 GMT00:00 when creating a new Date object. When retrieving time information like year and hour, your local timezone is applied to determine the correct local time. Thus, even though your date object reads Date(2020,02,08,02,00,00), JS still treats it as Standard (winter) time since there's no 2am in Daylight Savings time. The tick value remains the same as 03:00:00 GMT-00:40, with the hour being localized to your timezone. Essentially, 02:00:00 GMT-05:00 and 03:00:00 GMT-04:00 are indistinguishable but represent different timezones/daylight saving periods.

In Fall, your date/time parsing logic may face challenges as the hour between 1 and 2 occurs twice. According to TimeAndDate, on 11/1 at 2am, the clock shifts back to 1am. Therefore,

Date(2020, 10, 01, 01, 30, 00)

cannot definitively identify whether it's still EDT or already EST without specifying the current timezone/offset to UTC.

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