Function now 

pub fn now() -> Duration

Monotonic time since an unspecified epoch.

let t0 = nros::time::now();
// ... work ...
let dt = nros::time::now() - t0;

Reads the platform port’s nros_platform_clock_ns (RFC-0073) — on a hosted build too, which is the point: this and the executor must be ONE clock. What the low digits are worth is per-port; ask nros_platform_clock_resolution_ns.