Database
[{user:123,goal:"start_command",time:ISODate("2023-06-15T11:59:03.711Z")},{user:234,goal:"start_command",time:ISODate("2023-06-16T12:06:42.976Z")},{user:67,goal:"enroll_button",time:ISODate("2023-06-18T12:11:32.078Z")}]
Query
db.collection.aggregate([{"$addFields":{"time_unit":"day"}},{"$addFields":{"trunc_time":{$dateTrunc:{"date":"$time","unit":"$time_unit"}}}},{"$densify":{"field":"trunc_time","range":{"step":1,"unit":"$time_unit","bounds":"full"}}}])