Help with IIf query

[tezread]

I have a query where one custom field is working out the number of days:HoldDays: DateDiff("d",qry_cardiology1.HoldStartDateTime1,qry_cardiology1.HoldEndDateTime1)My problem is that sometimes there will not be a value for HoldStartDateTime or HoldEndStartTime so I want to add a condition that tests to see if there are values and if not just have '0'How could I do this guysCheersTez