To add to this, this is why we have those funny functions log1p() and expm1() in the standard library.

To add micromorts:

    double add_micromorts(double x, double y) {
        return -1e6 * expm1(
            -log1p(x * 1e-6) - log1p(y * 1e-6)
        );
    }

This is not the most accurate way of doing things, it is much simpler to

    return x + y - x * y * 1e-6;

But the formula with expm1 and log1p lets you easily do things like multiply 10 micromorts/day by 10.5 days.