How can I roll up / calculate dates based on in items linked "contributors"

Hi,

I have a task with date fields StartDate and EndDate. The task has subtasks linked via contributes/tracks. The subtasks also has StartDate and EndDate.

How can I make the "parent" task to automatically have the earliest date from it's "child" tasks StartDates in its own StartDate and for EndDate vice versa?

Hence:

Task1 StartDate: 1/1/1900 EndDate: 1/12/1900

\- tracks -> Task2 StartDate: 1/1/1900 EndDate 1/9/1900

\- tracks -> Task3 StartDate: 3/3/1900 EndDate 1/12/1900

StartDate and EndDate of Task1 should be automatically caluclated/rolled up.

Thanks for any hints.

Franca