why should a timestamp reduce the amount of logical IO? As I understand you limit your query based on a range of 10 days so I don't see the benefit of a timestamp.

Also when accessing 1/6 of all rows (10 days/ total of 60 days) it is probably faster to do a fts then accessing via an index.

Instead of using a index you may partition your table at a daily level. This way you would probably get the best response time for your query.