Parsing ISO week date incorrect for some years

[mrkjhnstn]

There appears to be a bug in ISO year-week parsing for some years: 2012 is an example:

var x = moment.utc('2012-W01');

This parses 1 week earlier than it should, and formats as follows:

x.format("GGGG-[W]WW")  -> '2011-W52'

but the correct string should be '2012-W01', same as input.

Here is the moment object:

{ _isAMomentObject: true,
  _useUTC: true,
  _isUTC: true,
  _l: undefined,
  _i: '2012-W01',
  _f: 'GGGG-[W]WW ',
  _strict: undefined,
  _pf: 
   { empty: false,
     unusedTokens: [],
     unusedInput: [],
     overflow: -1,
     charsLeftOver: 0,
     nullInput: false,
     invalidMonth: null,
     invalidFormat: false,
     userInvalidated: false,
     iso: true },
  _a: 
   [ 2011, 11, 26, 0, 0, 0, 0 ],
  _w: { GG: '2012', W: '01' },
  _dayOfYear: 360,
  _d: Sun Dec 25 2011 16:00:00 GMT-0800 (PST),
  _offset: 0 }

Looking over a range of years 2000 to 2020, this happens only for 3 years:

var yr;
for (yr = 2000; yr < 2021; ++yr) {
    var strIn = yr.toString() + '-W01';
    var strOut = moment.utc(strIn).format('GGGG-[W]WW');
    var same = (strIn == strOut);
    if (!same) {
        console.log(strIn + ' differs from formatted version ' + strOut);
    }
}

produces:

2006-W01 differs from formatted version 2005-W52
2012-W01 differs from formatted version 2011-W52
2017-W01 differs from formatted version 2016-W52

[ichernev]

@mrkjhnstn thank you for the report. It was really helpful in finding and fixing the bug.