Although a large, fixed precision type is probably an easier default, in practice a floating point variable will work at least as well as a 53 bit fixed integer

Although a large, fixed precision type is probably an easier default, in practice a floating point variable will work at least as well as a 53 bit fixed integer scaled to the range you're interested in. Inexactness through rounding isn't a big deal, because you rarely care about exact equality for inexact measures like time.