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I'm not much of a DBA. I use MySQL and PG every now and again. Reading the above posts, I had a thought, and I assume I'm wrong, and if you can, I'd love it if someone would explain what's wrong with my thought.
What about creating yet another index with the timestamp in the format you need? It seems to me that you'd be trading a bunch of space for exactly the performance you're looking for. It also seems to me that the DBs that allow you to index on a function might just be doing exactly this under the covers.