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Javascript – Why does moment.startOf go back 1 hour?

Edit: My local timezone is UTC+1

  • I have this date string: 2024-07-01T00:00:00.000Z.

  • This date string has a 'Z' at the end which denotes that it is in UTC (If I understand correctly).

When I run

moment('2024-07-01T00:00:00.000Z').toISOString()

-> '2024-07-01T00:00:00.000Z' // returns

Cool, that’s what I expect.

But when I want to get the start of that day for that moment object:

moment('2024-07-01T00:00:00.000Z').startOf('day').toISOString();

=> '2024-06-30T23:00:00.000Z' // returns

Question: Why is it returning an hour earlier, shifting a day behind my original date which was denoted in UTC?

I suspect that it has to do with my UTC offset of +1. It is for some reason, assuming that '2024-07-01T00:00:00.000Z' is 1 hour ahead of UTC?

Many thanks!

Btw, my moment version is 2.29.4

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2

Answers


  1. 0

    By default, moment parses and displays in local time. If you want to parse date as UTC, you have use moment.utc. If you want to pass offset in time, then you need to use moment.parseZone that will resolve to UTC if no offset given. You can also switch between UTC and local mode calling .local and .utc on your instance. And if you want to check if you are working with UTC date or not, you can call creationData().isUTC that will give you current mode.

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  2. 0

    You are in a time zone one hour ahead of UTC. So if you do

    moment('2024-07-01T00:00:00.000Z')

    You are parsing the given timestamp (where the Z defines it is in UTC to a instance of moment. As you are in UTC+1 and moment uses localtime per default, this is 2024-07-01T01:00:00 in your localtime.

    When you now do .startOf('day') you are going to the start of the day 2024-07-01T00:00:00.000, again in local time. And finally .toISOString() creates a UTC representation of this timestamp, which is of course 2024-06-30T23:00:00.000Z

    If you want your resulting moment instance to be in UTC, use moment.utc(...) instead.

    Btw: momentjs is considered deprecated and is not maintained anymore and its authors suggest to not use it in new projects anymore.

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