Sorry, I thought completion should be after registry:
SELECT [dateToLandRegistry]-[dateOfCompletion] AS Expr1, [completion matters].[completion ID], [completion matters].clientName, [completion matters].completedBy, [completion matters].dateOfCompletion, [completion matters].dateToLandRegistry
FROM [completion matters]
WHERE ((([dateToLandRegistry]-[dateOfCompletion])>30));