DateTime filter on AOT query

I am building a report that shows all the LedgerJournalEntry in a date range, using parameters DateFrom to DateTo. I built an AOT query and added the field PostedDateTime as a range. How do I set a value for this range so it searches for :

paramDateFrom<PostedDatetime<paramDateTo

if I leave the value as empty it matches for the exact date. I can put in ">8/20/2012" but I want to use the value that user entered.

I am not using dynamic filter on the report so user cannot enter something like ">8/20/2012". They have to pick a date from 2 calendar controls, one for DateFrom one for DateTo.

Thanks

Holly