I've trying to make a simple task list in Microsoft Excel 2012:

```
Task# Parent Task# Task Description Days Due Date
1 Catch the llama 1
2 1 Shave the llama 1
3 2 Paint the llama 3 31/3/2012
```

In the "Due Date" column I want to be able to do the following calculation:

If a child task has a parent task then subtract the number of days the parent task will take from it's due date for the due date of the child task.

So "Shave the llama" would have a "Due Date" of "28/3/2012" as it would subtract the 3 days it takes to "Paint the llama", which is it's parent task.

To achieve this I need to search the "Parent Task#" column and then reference additional cells on the matched row.

How would I achieve this?