`YYYY-WW` format of 29 Dec 2019 is `2019-01`

[fengelniederhammer]

Describe the bug

import dayjs from 'dayjs/esm';
import isoWeek from 'dayjs/esm/plugin/isoWeek';
import advancedFormat from 'dayjs/esm/plugin/advancedFormat';

dayjs.extend(isoWeek);
dayjs.extend(advancedFormat);

const firstDay = dayjs().year(2020).month(1).date(4).isoWeek(1).startOf('isoWeek');

firstDay.toString(); // Sun, 29 Dec 2019 23:00:00 GMT - that seems ok
firstDay.format('YYYY-WW'); // 2019-01 - this is definitely not correct.

Expected behavior YYYY-WW of 29 Dec 2019 should be 2020-01 (according to https://www.epochconverter.com/weeks/2019)

Information

• Day.js Version 1.11.10
• OS: Ubuntu
• Browser none/Node
• Time zone: CEST (UTC +2)