Does that mean you are looking for a Designer solution? If so, we can move this topic to the Semantic Layer forum.
That said, if your dates are true datetime fields, the Oracle solution is rather simple
(tbl.FundDate - tbl.ReqDate) * 86400
Simple Oracle date math returns days … any fractional days as a decimal (.5 = 12 hours, for example) … multiply that by 86400 (number of seconds in one day) and that’s the answer.