SET SETUNKNOWNRESULTTONULL OFF | ONEmpty concept it is an obsolete concept that arrives from the origins of XBase.
CLEAR d1 = DATE() d2 =DATE(2005,2,31) && return a empty date because the date don't exists ? d1#d2,d1-d2 && are not equal, but difference is zero !! ( empty date go to a empty number ) * workaround, map empty to NULL d2 =EVL(DATE(2005,2,31),CAST(NULL AS D)) ? d1#d2,d1-d2 * effect of empty on computation dRef = {^2005/1/1} CREATE CURSOR TESTDATE (DD D NULL) INSERT INTO TESTDATE VALUES (DATE(2005,2,31)) && this is a unknown date INSERT INTO TESTDATE VALUES (dRef+1) SELECT AVG(DD-m.dRef) AvgDiff FROM TESTDATE && Average it is mistaken USE ZAP IN TESTDATE * WORKAROUND INSERT INTO TESTDATE VALUES (EVL(DATE(2005,2,31),NULL)) INSERT INTO TESTDATE VALUES (dRef+1) SELECT AVG(DD-m.dRef) AvgDiff FROM TESTDATEFabio