You can use a
combination of Finish to Finish (FF) links and
hammocks: use a FF link from the end of the original
task (task A) to the end of the bit to subtract (task
B). Make sure the task B is set to "Never stretch" (on
Task Details, General tab).
/>This will effectively take some time off the task
A. However, it doesn't really make the task A shorter,
it just counts backwards from its finish. The final
step is to use a hammock to measure the final result -
this hammock has a SS link from the original task A,
and a FF link from the task B.
/>I hope this helps!