[{"datetime":ISODate("2022-10-16")},{"datetime":ISODate("2022-10-17")},{"datetime":ISODate("2022-10-18")},{"datetime":ISODate("2022-10-19")},{"datetime":ISODate("2022-10-20")}]

db.collection.aggregate([{"$facet":{"smaller":[{$match:{datetime:{$lt:ISODate("2022-10-18")}}},{$sort:{datetime:-1}},{$limit:1}],"within":[{$match:{datetime:{$gte:ISODate("2022-10-18"),$lte:ISODate("2022-10-19")}}}],"larger":[{$match:{datetime:{$gt:ISODate("2022-10-19")}}},{$sort:{datetime:1}},{$limit:1}]}},/** wrangling*/{$project:{all:{"$setUnion":["$smaller","$within","$larger"]}}},{"$unwind":"$all"},{"$replaceRoot":{"newRoot":"$all"}}])