Database
[{"_id":"ID1","seen_at":[ISODate("2018-12-27T17:00:00.000Z"),ISODate("2019-01-01T01:00:00.000Z")]},{"_id":"ID2","seen_at":[ISODate("2019-01-01T00:00:00.001Z")]},]
Query
db.collection.aggregate([{$unwind:"$seen_at"},{"$match":{seen_at:{"$gt":ISODate("2019-01-01T00:00:00.000Z"),"$lt":ISODate("2019-01-01T00:00:00.010Z")}}},{$group:{"_id":"$_id","seen_at":{$push:"$seen_at"}}}])