mongodb index strategy for range query with different fields

Almost all my documents include 2 fields, start timestamp and final timestamp. And in each query, I need to retrieve elements which are in selected period of time. so start should be after selected value and final should be before selected timestamp.

query looks like

db.collection.find({start:{$gt:DateTime(...)}, final:{$lt:DateTime(...)}})

So what is the best indexing strategy for that scenario?

By the way, which is better for performance - to store date as datetimes or as unix timestamps, which is long value itself