LibWeb: Bring performance.now() closer to spec

Author: https://github.com/stelar7
Commit: f6991a2955
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1639
Reviewed-by: https://github.com/tcl3

Userland/Libraries/LibWeb/HighResolutionTime/TimeOrigin.cpp

@@ -14,9 +14,32 @@ namespace Web::HighResolutionTime {
 // https://w3c.github.io/hr-time/#dfn-get-time-origin-timestamp
 DOMHighResTimeStamp get_time_origin_timestamp(JS::Object const& global)
 {
-    // FIXME: Implement this.
     (void)global;
-    return 0;
+
+    // To get time origin timestamp, given a global object global, run the following steps, which return a duration:
+    // FIXME: 1. Let timeOrigin be global's relevant settings object's time origin.
+    auto time_origin = 0;
+
+    // Each group of environment settings objects that could possibly communicate in any way
+    // has an estimated monotonic time of the Unix epoch, a moment on the monotonic clock,
+    // whose value is initialized by the following steps:
+
+    // !. Let wall time be the wall clock's unsafe current time.
+    struct timeval tv;
+    gettimeofday(&tv, nullptr);
+    auto wall_time = tv.tv_sec * 1000.0 + tv.tv_usec / 1000.0;
+
+    // 2. Let monotonic time be the monotonic clock's unsafe current time.
+    auto monotonic_time = unsafe_shared_current_time();
+
+    // 3. Let epoch time be monotonic time - (wall time - Unix epoch)
+    auto epoch_time = monotonic_time - (wall_time - 0);
+
+    // 4. Initialize the estimated monotonic time of the Unix epoch to the result of calling coarsen time with epoch time
+    auto estimated_monotonic_time = coarsen_time(epoch_time);
+
+    // 2. Return the duration from the estimated monotonic time of the Unix epoch to timeOrigin.
+    return estimated_monotonic_time - time_origin;
 }
 
 // https://w3c.github.io/hr-time/#dfn-coarsen-time