I found this suggestion on another post:
IF isNull({HPD_Help_Desk.Last Resolved Date})
THEN
DateDiff ("d",{HPD_Help_Desk.Reported Date} ,date(1900, 01, 01))
ELSE
DateDiff ("d",{HPD_Help_Desk.Reported Date} ,{HPD_Help_Desk.Last Resolved Date} )
// date(1900, 01, 01)
I tried that, but I am...