Invalid day parameter for DATE() function does not generate an error in a query but produces empty date. EVL() or IIF in VFP7 and earlier can be used to replace Feb. 29 with Feb. 28 in such case assuming that all DOBs are valid dates (not empty or null).

SELECT * ;
	FROM People ;
	WHERE ;
			EVL(DATE(YEAR(ldStartDate), MONTH(dob), DAY(dob)),  ;
				DATE(YEAR(ldStartDate ), MONTH(dob), DAY(dob-1))) ; 
					BETWEEN ldStartDate AND ldEndDate ;
		OR ;
			EVL(DATE(YEAR(ldEndDate), MONTH(dob), DAY(dob)),  ;
				DATE(YEAR(ldEndDate), MONTH(dob), DAY(dob-1))) ;
					BETWEEN ldStartDate AND ldEndDate

>The date() function in your example would fail fail if a person happened to be born on Feb. 29 - I believe that I'll have to convert those dates to Feb. 28, using an iif().