diff --git a/lib/src/relative_clock.dart b/lib/src/relative_clock.dart
index c8560ab..f204441 100644
--- a/lib/src/relative_clock.dart
+++ b/lib/src/relative_clock.dart
@@ -1,24 +1,20 @@
-
 part of clock;
 
 typedef Duration _Elapsed();
 
-class _RelativeClock implements Clock {
-  
+class _RelativeClock implements _SystemClock {
   final DateTime _initialTime;
   final _Elapsed _elapsed;
   final int _frequency;
-  
-  _RelativeClock({
-    DateTime initialTime, 
-    Duration elapsed(),
-    int frequency: 1000000
-  }) 
-      : _initialTime = initialTime == null ? _zonedClock.value.now : initialTime,
+
+  _RelativeClock(
+      {DateTime initialTime, Duration elapsed(), int frequency: 1000000})
+      : _initialTime =
+            initialTime == null ? _zonedClock.value.now : initialTime,
         _elapsed = elapsed == null ? (() => Duration.ZERO) : elapsed,
         _frequency = frequency;
-  
+
   DateTime get now => _initialTime.add(_elapsed());
-  Stopwatch getStopwatch() => new FakeStopwatch(() => 
-      _elapsed().inMicroseconds * _frequency ~/ 1000000, _frequency);
+  Stopwatch getStopwatch() => new FakeStopwatch(
+      () => _elapsed().inMicroseconds * _frequency ~/ 1000000, _frequency);
 }