如果您可以确定输入日期字符串的格式，而您只是想获取唯一的YYYYMMDD的计数，则只需$ project子字符串，然后分组:

If you can be assured of the format of the input date string AND you are just trying to get a count of unique YYYYMMDD, then just $project the substring and group on it:

var data = [
        { "name": "buzz", "d1": "2015-06-16T17:50:30.081Z"},
        { "name": "matt", "d1": "2018-06-16T17:50:30.081Z"},
        { "name": "bob", "d1": "2018-06-16T17:50:30.081Z"},
        { "name": "corn", "d1": "2019-06-16T17:50:30.081Z"},
      ];

db.foo.drop();
db.foo.insert(data);

db.foo.aggregate([
     { "$project": {
         "xd": { "$substr": [ "$d1", 0, 10 ]}
         }
 },
 { "$group": {
             "_id": "$xd",
             "n": {$sum: 1}
         }
     }
              ]);

{ "_id" : "2019-06-16", "n" : 1 }
{ "_id" : "2018-06-16", "n" : 2 }
{ "_id" : "2015-06-16", "n" : 1 }