Browser API's microsecond precision is lost

[rofferom]

I'm sharing my analysis about an issue I have encountered. Although I manage to fix my specific usecase, I'd like to have some feedback before trying to open a PR to propose a more global fix.

Context

I wanted to make some performance measurement, and I realized that I had a milliseconds precision.

My test basically relies on this kind of code structure:

let a = Instant::now();

....

let b = Instant::now();
let d = b.duration_since(a).as_micros();
log::info!("{:?} => {:?}: {}", a, b, d.as_micros());

Here is the kind of result is get:

Instant { inner: 1606.7599999904633 } => Instant { inner: 1614.7899999916553 }: 8000

performance.now() theoretical precision

According to https://developer.mozilla.org/en-US/docs/Web/API/Performance/now#performance.now_vs._date.now:

the timestamps returned by performance.now() are not limited to one-millisecond resolution. Instead, they represent times as floating-point numbers with up to microsecond precision

Dirty prototyping

I have patched quickly the library to try to keep the millisecond precision in my specific codepath:

diff --git a/src/std.rs b/src/std.rs
index 7c88189..fee6573 100644
--- a/src/std.rs
+++ b/src/std.rs
@@ -112,9 +112,9 @@ impl Sub<Instant> for Instant {
     type Output = Duration;
 
     fn sub(self, other: Instant) -> Duration {
-        let ms = self.inner - other.inner;
-        assert!(ms >= 0.0);
-        Duration::from_millis(ms as u64)
+        let d = self.inner - other.inner;
+        assert!(d >= 0.0);
+        Duration::from_micros((d * 1000.0) as u64)
     }
 }

Result:

Instant { inner: 2657.5049999952316 } => Instant { inner: 2666.9650000035763 }: 9460