Yeah, fixed point is different from rational, and all those examples you gave sound to me like fixed point. And that can be implemented efficiently without dedi

Yeah, fixed point is different from rational, and all those examples you gave sound to me like fixed point. And that can be implemented efficiently without dedicated hardware support: the denominator is fixed, and you store the numerator as an integer. A 1/3 off discount on a $10 item is $6.67 (or $6.66 if rounding in the customer's favor), not $10/3. (Except datetime, did you mean timestamp? A timestamp is an instant in time, and it often makes sense to store it in high precision because you're saying exactly when something happened. A datetime is for communicating between humans who are using some particular calendar; it rarely makes sense to have more than minute precision.)