From The Economist on compound interest savings and intergalactic travel:

Starship enterprises

Dismal scientists also like speculating about space flight

On a starship travelling near the speed of light, a handful of years might pass for the crew while hundreds passed on a planet it was travelling from or to. That makes calculating interest rates hard, since savers could pay starship captains to carry them forward in time, to reap the benefits of centuries of accumulated interest

The economics of interstellar flight: Starship enterprises | The Economist