Mongodb日期时间到字符串（yyyy/mm/dd）

我有以下mongodb输出。我希望我的datetime采用以下格式：yyyy/mm/dd，以便在以后的阶段进行比较。我尝试了很多东西，以下是我认为最适合的两个：

输出：

另一种尝试：

{
    "collection": "device_color",
    "query": {
        "device_id": {"$in": [587,579,1241,543]},
        "datetime": {
            "$gte": {
                "$humanTime": "2020-09-01"
            }
        }
    },
    "fields": {
        "datetime": 1,
        "device_id": 2,
        "color": 3
    },
    "sort": [
        {
        "name": "device_id",
        "direction": -1
    },
        {
        "name": "datetime",
        "direction": 1
    }
    ],
    "$addFields": {
        "sentDateString": { 
            "$dateToString": { 
                "format": "%Y-%m-%d", 
                "date": "$datetime" 
            } 
        }
    }
}

我也试过这样的方法：

{
    "collection": "device_color",
    "query": {
        "device_id": {"$in": [587,579,1241,543]},
        "datetime": {
            "$gte": {
                "$humanTime": "2020-09-01"
            }
        }
    },
    "$project": {
            "datetime": { "$dateToString": { "format": "%Y-%m-%d", "date": "$datetime" } },
            "device_id": 2,
            "color": 3
    },
    "sort": [
        {
        "name": "device_id",
        "direction": -1
    },
        {
        "name": "datetime",
        "direction": 1
    }
    
    ]
}

{
    "collection": "device_color",
    "query": {
        "device_id": {"$in": [587,579,1241,543]},
        "datetime": {
            "$gte": {
                "$humanTime": "2020-09-01"
            }
        }
    },
    "$project": {
            "datetime": { "$dateToString": { "format": "%Y-%m-%d", "date": "$datetime" } },
            "device_id": 2,
            "color": 3
    },
    "sort": [
        {
        "name": "device_id",
        "direction": -1
    },
        {
        "name": "datetime",
        "direction": 1
    }
    
    ]
}