How to Auto Calculated time in SQL Database Select Column

Question

my database table is tableone.
for example......

INSERT INTO tableone(Name,EntryDate,ServiceDate) VALUES('MG MG', '1/1/2012' , ' ' )
INSERT INTO tableone(Name,EntryDate,ServiceDate) VALUES('KO KO', '1/3/2012' , ' ' )
INSERT INTO tableone(Name,EntryDate,ServiceDate) VALUES('John', '1/2/2012' , ' ' )

Now,We inserted data to tableone but ServiceDate Column is empty.This ServiceDate Column is
Now time(CURDATE)-EntryDateColumn values Inserted and Update ServiceDate Column.This ServiceDate Column is ever auto calculation and auto update inserted to ServiceDate Column.Please Help Me...Thank You...