Allow treating ObjectId type as date in $project phase

Since ObjectId() encompasses an approximation of insertion date it would be helpful if _id/things of ObjectId() type could be operated/projected the same way as Datetime type object.

One question would be whether ObjectId could be operated on directly via date/interval arithmetic. I.e. whether it can be used anywhere a date can be used returning a standard Datetime type as a result from say {$subtract:["$_id",86400000]} which can only return ts of one day before that ObjectId's timestamp.