First of, never fetch DateTime.Now more than once: it leads to intermittent problem around midnight which can really mess you up. Fetch it once, and use it for all checks.
Then try:
DateTime now = DateTime.Now;
var receiveDetails = objDB.GetAllReceivedDetails().Where(a => a.ReceiveDate.Value.Date == now.Date && (now - a.ReceiveDate.Value).TotalHours <= 1);