Is it better to get $100 today or $110 in one years time? The answer’s not straightforward because you could use $100 today to potentially earn more than $10 over 12 months. How do you know what to do? For that, you need to know how to calculate a rate of return or use it to compare amounts of money at different times. Hold on for a bit, we’re bending the time—space (money?) continuum.