incorrect format of week

[wenqi73]

Hi, I use yyyy-ww to format date, but the return week is 01.

import { format } from 'date-fns';
format(new Date(2019, 11, 30), 'yyyy-ww');
// 2019-01

[SoniaLahi]

This behavior is correct. Indeed, Monday 30th December 2019 is a Monday, the first Monday of the first week of 2020.

You can see more here : http://week-number.net/calendar-with-week-numbers-2019.html

I have tried with these values :

new Date(2019, 11, 28)  // Saturday  -> output 52
new Date(2019, 11, 29) // Sunday -> output 52 if first day of week is set to Monday, otherwise by default (Sunday) output is 01
new Date(2019, 11, 30) // Monday -> output 01
new Date(2019, 11, 31) // Tuesday -> output 01

[wenqi73]

@kossnocorp Could you please check this issue?

[kossnocorp]

@wenqi73 @SoniaLahi did answer your question (thanks, Sonia!). You maybe want to use ISO-week numbering II instead. Or not. It highly depends on your use-case. I recommend you to read about it more to figure out what you actually need (you can start here: https://en.wikipedia.org/wiki/Week#Week_numbering).

If you actually need ww, you must use YYYY-ww which would produce (with II you must use RRRR):

format(new Date(2019, 11, 30), 'YYYY-ww');
//=> 2020-01

[wenqi73]

@kossnocorp Thanks for your explanation! I have a little confusing, the usage of ww is similar to ISO-week, but it regards Sunday as the beginning of the week(ISO-week uses Monday as the beginning), seems it's not a standard format.