A network of locally interacting agents can be thought of as performing a distributed computation. But not all computations can be faithfully distributed. This technical note investigates which global, linear transformations can be computed in finite time using local rules with time varying weights, i.e., rules which rely solely on information from adjacent nodes in a network. The main result states that a linear transformation is computable in finite time using local rules if and only if the transformation has positive determinant. An optimal control problem is solved for finding the local interaction rules, and simulations are performed to elucidate how optimal solutions can be obtained.