Adding Dates together in DD/MM/YYYY format

Hi All,

New here, got tasked with something at work today which i am struggling with.

Ive got a TOTAL Completion date and completion dates for sub-jobs. Example:

Completion date
12/01/2010

Sub-Job Completion date
15/01/2010

So it would be overdue by 3days. (making the total completion date = 18/01/2010)

How can I make the completion date change to reflect this?

So, when the Sub-Job completion date changes the completion date also changes!!

I think its using the differnce of days between them but i need it to use the WHOLE date (years months and years)... But the numbers are hurting my head

Hope some one can help!!

Cheers,
Chris