Add variants of $week, $year, $dayOfWeek that conform to the ISO-6801 spec*

{_id: 0, date: ISODate("2016-01-01T00:00:00Z")}  // Thu, Jan 1, 2016

db.coll.aggregate([{

    $project: {

        dayOfWeek: {$dayOfWeek: "$date"},

        isoDayOfWeek: {$isoDayOfWeek: "$date"},

        year: {$year: "$date"},

        isoYear: {$isoYear: "$date"},

        week: {$week: "$date"},

        isoWeek: {$isoWeek: "$date"},

        isoString: {$dateToString: {format: "%G-W%V-%u", date: "$date"}}

    }

}])

{

  _id: 0,

  dayOfWeek: 6,  // 1 is Sunday, 7 is Saturday

  isoDayOfWeek: 5,  // 1 is Monday, 7 is Sunday

  year: 2016,

  isoYear: 2015,  // 2016 starts on Jan 4th under ISO calendar.

  week: 0,

  isoWeek: 53,

  isoString: "2015-W53-5"  // $dateToString is also modified to support

                           // the concepts above.

}