Wasm: allow Instant to represent instants in past

src/wasm.rs

@@ -16,7 +16,14 @@ impl Eq for Instant {}
 impl Instant {
     #[inline]
     pub fn now() -> Self {
-        Instant(duration_from_f64(now()))
+        // `performance.now()` starts counting from the creation of the page, which mean that
+        // instants before the page start are negative.
+        //
+        // So, to allow represing past instants using a `Duration` (that can't represent negative
+        // values), the duration will be increased by 2^63 seconds.
+
+        let duration = duration_from_f64(now());
+        Self(duration + Duration::from_secs(1 << 63))
     }
 
     #[inline]

tests/wasm.rs

@@ -55,3 +55,10 @@ fn test_system_time() {
         .is_err());
 }
 
+#[wasm_bindgen_test]
+fn test_past() {
+    let hundred_sec = Duration::from_secs(100);
+    let hundred_seconds_ago = Instant::now() - hundred_sec;
+    assert!(hundred_seconds_ago.elapsed() >= hundred_sec);
+}
+