(1,1,'07-04-2014','08-04-2014'),
(2,2,'07-04-2014','07-04-2014'),
(3,3,'08-04-2014','10-04-2014'),
(4,4,'09-04-2014','10-04-2014')

SELECT AVG(DATEDIFF(DD,'19700101',dateField)) AS avgDays

SELECT DATEDIFF(MINUTE, '1901-1-1', patientEntryDate)