问mongodb系统当前日期(yyyy-mm-dd)

EN

db.hello.aggregate([
    { $match: { "name": "btb" } },
    { $where: function() { 
        today = new Date(); 
        today.setHours(0,0,0,0);
        return (this.date.getTimestamp() >= today)
    }},
    { $group: {
        _id: { articles: "$articles" },
        total: { $sum: 1 }
    }},
    { $project: {
        "articles": 1,
        "total": 1
    }},
    { $sort: { "total":-1 } }
]) 

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Unable to execute the selected commands

Mongo Server error (MongoCommandException): Command failed with error 16436: 'Unrecognized pipeline stage name: '$where'' on server localhost:27017. 

The full response is:
{ 
    "ok" : 0.0, 
    "errmsg" : "Unrecognized pipeline stage name: '$where'", 
    "code" : NumberInt(16436)
}

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