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
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 an 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 localtime. 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.