Counting child documents by full date in MongoDB

group_child_by_date.js

// grab article.comments, group by date (roll times up to full days), count up instances to get full counts per day.
// gotta be a better way...

db.Article.aggregate( 
    {$unwind: '$Comments'},
    {$group: { _id: '$Comments.DateCreated', count : {$sum: 1 }}},
    {$match: { '_id' : { $gte: ISODate('2014-02-21 00:00:00'), $lte: ISODate('2014-02-28 23:59:59') } }},
    {$project: { _id: 0, count: 1, 'date': '$_id', h: { $hour: '$_id' }, m: { $minute: '$_id'}, s: { $second: '$_id' }, ml: {$millisecond: '$_id'} } },
    {$project: { count: 1, 'dt': { "$subtract" : [ "$date", { "$add" : [ "$ml", { "$multiply" : [ "$s", 1000 ] }, { "$multiply" : [ "$m", 60, 1000 ] },  { "$multiply" : [  "$h", 60, 60, 1000  ] } ] } ] } }},
    {$group: { _id: '$dt', count: { $sum: '$count' } } },
    {$sort: { _id: 1 } }
)