Add inaccurate time option

For environments where `performance.now()` is not available such as
Cloudflare Workers.

Cargo.toml

@@ -12,6 +12,7 @@ edition = "2018"
 [features]
 now = [ "time" ]
 wasm-bindgen = ["js-sys", "wasm-bindgen_rs", "web-sys"]
+inaccurate = []
 
 [dependencies]
 cfg-if = "1.0"

README.md

@@ -55,6 +55,26 @@ fn main() {
 }
 ```
 
+-----
+
+### Using `instant` for a WASM platform where `performance.now()` is not available.
+This example shows the use of the `inaccurate` feature.
+
+_Cargo.toml_:
+```toml
+[dependencies]
+instant = { version = "0.1", features = [ "wasm-bindgen", "inaccurate" ] }
+```
+
+_main.rs_:
+```rust
+fn main() {
+    // Will emulate `std::time::Instant` based on `Date.now()`.
+    let now = instant::Instant::new();
+}
+```
+
+
 -----
 
 ### Using `instant` for any platform enabling a feature transitively.

src/wasm.rs

@@ -101,18 +101,27 @@ pub fn now() -> f64 {
     use stdweb::unstable::TryInto;
 
     // https://developer.mozilla.org/en-US/docs/Web/API/Performance/now
+    #[cfg(not(feature = "inaccurate"))]
     let v = js! { return performance.now(); };
+    #[cfg(feature = "inaccurate")]
+    let v = js! { return Date.now(); };
     v.try_into().unwrap()
 }
 
 #[cfg(feature = "wasm-bindgen")]
 pub fn now() -> f64 {
-    use wasm_bindgen_rs::prelude::*;
-    use wasm_bindgen_rs::JsCast;
-    js_sys::Reflect::get(&js_sys::global(), &JsValue::from_str("performance"))
-        .expect("failed to get performance from global object")
-        .unchecked_into::<web_sys::Performance>()
-        .now()
+    #[cfg(not(feature = "inaccurate"))]
+    let now = {
+        use wasm_bindgen_rs::prelude::*;
+        use wasm_bindgen_rs::JsCast;
+        js_sys::Reflect::get(&js_sys::global(), &JsValue::from_str("performance"))
+            .expect("failed to get performance from global object")
+            .unchecked_into::<web_sys::Performance>()
+            .now()
+    };
+    #[cfg(feature = "inaccurate")]
+    let now = js_sys::Date::now();
+    now
 }
 
 // The JS now function is in a module so it won't have to be renamed

tests/wasm.rs

@@ -10,6 +10,9 @@ wasm_bindgen_test_configure!(run_in_browser);
 #[wasm_bindgen_test]
 fn test_instant_now() {
     let now = Instant::now();
+    #[cfg(feature = "inaccurate")]
+    while now.elapsed().as_millis() == 0 {}
+    #[cfg(not(feature = "inaccurate"))]
     assert!(now.elapsed().as_nanos() > 0);
 }