OK I have found a solution - as I didn't actually need the date to show anywhere I just needed to show length of service at the point of leaving or todate I have included date() in the equation thus
=IIf([Date Of Leaving]<Date()," " & Int(DateDiff('m',[Forms]![Staff List]![Date of Employment],[Forms]![Staff List]![Date of Leaving])/12) & " yrs " & (DateDiff('m',[Forms]![Staff List]![Date of Employment],[Forms]![Staff List]![Date of Leaving]) Mod 12) & " mths"," " & Int(DateDiff('m',[Forms]![Staff List]![Date of Employment],Date())/12) & " yrs " & (DateDiff('m',[Forms]![Staff List]![Date of Employment],Date()) Mod 12) & " mths "), bit longwinded but it does the job. Thanks for all your help.