The $dateToParts expression operator return a document with "isoYear" and "isoWeekYear" fields that confusingly correspond to the return values of the $isoWeekYear and $isoWeek expression operators, respectively. I think we should change the $dateToParts expression operator to use the field names of "isoWeekYear" and "isoWeek" to match the behavior of the existing $isoWeekYear and $isoWeek expression operators.

Note: We'll want to change $dateFromParts to take the "isoWeekYear" and "isoWeek" field names instead too.

> db.mycoll.aggregate([{$addFields: {
  parts: {$dateToParts: {date: "$date", iso8601: true}},
  isoWeekYear: {$isoWeekYear: "$date"},
  isoWeek: {$isoWeek: "$date"},
}}]).toArray()
[
	{
		"_id" : ObjectId("59ce4f35ac2831cef80ed375"),
		"date" : ISODate("2017-09-29T13:48:37.665Z"),
		"parts" : {
			"isoYear" : 2017,
			"isoWeekYear" : 39,
			"isoDayOfWeek" : 5,
			"hour" : 13,
			"minute" : 48,
			"second" : 37,
			"millisecond" : 665
		},
		"isoWeekYear" : NumberLong(2017),
		"isoWeek" : 39
	}
]