Database

[{"_id":1,"datetime":ISODate("2021-07-11T00:00:00.000Z")},{"_id":2,"datetime":ISODate("2021-07-11T00:16:00.000Z")},{"_id":3,"datetime":ISODate("2021-07-11T00:00:00.000Z")}]

Query

db.collection.aggregate([{"$addFields":{"datetime":{"$add":[ISODate("1970-01-01"),{"$multiply":[{$floor:{"$divide":[{$subtract:["$datetime",new Date("1970-01-01")]},900000]}},900000]}]}}},{$group:{_id:"$datetime",cnt:{$sum:1}}}])

Result