Expected date of delivery tells you by which date you should have the gravida at her best for a safe confinement.
Expected date of delivery is also called due date.
Though the short form of expected date of delivery is EDD, that of due date is not DD.
Only 50% gravidas deliver on the EDD.
50% gravidas deliver within a period of 15 days on either side of the EDD.
'Due Date' is an application that does something an obstetrician does by complex oral calculations.
An obstetrician may forget to make corrections for February and leap years, 'Due Date' doesn't. 
With some modifications, the application 'Due Date' can be used to find out the date for advance tax payment.
You have to enter the date of the gravida's last menstrual period in the text box provided for the same.
If the gravida's past cycles were irregular, you have to find the average duration of her past cyces for 'Due Date' to work.
If the gravida does not remember the date of her last menstrual period, you need ultrasonography for determination of gestational age. Deluxe edition of 'Due Date' helps you find EDD from ultrasonographic biometry.