But no matter what I entered as hours/minutes/seconds in my query I still got the same result.
There are good and bad news around this. The good news is that you can use the development api's to search with second resolution.
The bad news is that the Search Center UI disregards this, and when monitoring the queries received by FAST all hours/minutes/seconds are stripped from the query. They are adjusted for time zones as you can see below where 00:00:00 is turned into 23:00:00 of the day before the original query.
So if you want to do date range queries on an interval less than a day you have to use the development apis’s, and not the default Search page or query string.