Would a date clustered index speed up queries with date ranges? (SQL Server)
You ask if the clustered index is a waste: ideally a clustered index is unique, relatively small and does not change once created (see https://www.red-gate.com/simple-talk/sql/learn-sql-server/effective-clustered-indexes/ ) Your integer field is certainly small but my guess is it's not unique, although it's not possible to tell from the question.

To answer the question you could use an IDENTITY(1,1) as your clustered index key, then create a non-clustered index using your Unix time field. To make the queries faster you can INCLUDE commonly used columns in the non-clustered index, then if queries are covered by the index they need not even visit the main table.

Also if your last 3 months data could be defined in terms of particular date ranges, you could even build a filtered index to limit the size of the index and speed the searches. Based on experience with tables of 1m+ rows, if you are in control of the indexing you should be able to satisfy most queries in less than a second.