automatically insert into database

suppose i've one starting data(1/12/2013) and final date (26/12/2013). What i want is that to update the database with the difference between the dates every day...

means

start data  |  Final Data   |  Days Left
1/12/2012   | 26/12/2012    |  25                     <--------------     automatically done every day and updated in the database over writing the original values

Means after 2 days it will show  

start data  |  Final Data   |  Days Left
1/12/2012   | 26/12/2012    |  23 

Any help will be greately appreciated.....