new Date(...).toISOString() in JavaScript gives incorrect result on one day of the year-CodePudding

I'm building a calendar and I noticed that the date is incorrect on one day of the year.

I wanted to standardize dates to ISO 8601. Let's take 2022-03-27 for example:

const foo = new Date(2022, 2, 27).toISOString()

This gives 2022-03-27T00:00:00.000Z as one would expect.

However, the next day, 2022-03-28:

const bar = new Date(2022, 2, 28).toISOString()

This gives 2022-03-27T23:00:00.000Z - 27th March at 23:00.

Why does this happen?

CodePudding user response：

What's happening is that British Summer Time (Daylight Saving Time, aka DST) is kicking in between your two examples (specifically, it starts at 01:00 on 2022-03-27). toISOString always gives you UTC time (GMT) as you can tell from the Z at the end of the string, but your local time is an hour ahead of UTC after 01:00 on 2022-03-27 until DST ends at 02:00 on 30 October 2022.

In your first example, you're creating a Date with the local time of midnight on 2022-03-27, when your local time is (apparently) GMT 00:00 (like mine here in the UK). So toISOString gives you back 00:00 because your local time is GMT/UTC (that date/time is an hour before DST kicks in).

But in your second example, you're creating a Date in local time at midnight on 2022-03-28, when you're on DST. So you're offset from UTC by an hour (UTC is one hour behind you). Midnight 2022-03-28 UK time is 23:00 2022-03-27 UTC.

I don't know what you want to do with this stuff, but if you want to create a Date for midnight UTC on 2022-03-28, use new Date(Date.UTC(2022, 2, 28)).