weekYear(Number) jumps to the previous day of the week (and wraps around)

[xehpuk]

Description of the Issue and Steps to Reproduce:

For the German locale, weekYear(Number) does not preserve the day of the week, even if Number === weekYear(). It always jumps to the previous day and wraps around on Sunday.

import moment from 'moment'

moment.locale('de')

const date = moment('2016-01-01')

date.weekYear(2015) // 2015-12-31 Th
date.weekYear(2015) // 2015-12-30 We
date.weekYear(2015) // 2015-12-29 Tu
date.weekYear(2015) // 2015-12-28 Mo
date.weekYear(2015) // 2016-01-03 Su
date.weekYear(2015) // 2016-01-02 Sa
date.weekYear(2015) // 2016-01-01 Fr

// Workaround:
const day = date.day() // 5
date.weekYear(2015).day(day) // 2016-01-01 Fr

https://jsfiddle.net/mtvsr738/

Environment:

Chrome 57, Firefox 52 and Edge 38 on Windows 10

Other information that may be helpful:

Tue May 02 2017 21:08:56 GMT+0200 (Mitteleuropäische Sommerzeit)
2.5.2017, 21:08:56
-120
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.133 Safari/537.36
2.18.1

[icambron]

Looks like a bug to me. Should give the same week number and weekday. Would accept a patch.

[MTelling]

Seems to be an issue in other locales as well. I can repro in 'ca' and 'da' as well locally.

[xehpuk]

dayOfYearFromWeeks(year, week, weekday, dow, doy) seems to calculate a wrong dayOfYear.

moment([2015, 11, 27]).locale('de').weekYear(2015) leads to the call dayOfYearFromWeeks(2015, 52, 6, 1, 4) which then calculates dayOfYear = 360, while it should be 361.

[Nosfistis]

I am having the same issue, it behaves as if doy is ignored. @nikofil are you preparing a PR?