Wrapped the system clock in an interface so it can be replaced in tests.

api/net/sf/briar/api/plugins/duplex/DuplexPluginFactory.java

@@ -2,8 +2,10 @@ package net.sf.briar.api.plugins.duplex;
 
 import java.util.concurrent.Executor;
 
+import net.sf.briar.clock.Clock;
+
 public interface DuplexPluginFactory {
 
-	DuplexPlugin createPlugin(Executor pluginExecutor,
+	DuplexPlugin createPlugin(Executor pluginExecutor, Clock clock,
 			DuplexPluginCallback callback);
 }

api/net/sf/briar/api/plugins/simplex/SimplexPluginFactory.java

@@ -2,8 +2,10 @@ package net.sf.briar.api.plugins.simplex;
 
 import java.util.concurrent.Executor;
 
+import net.sf.briar.clock.Clock;
+
 public interface SimplexPluginFactory {
 
-	SimplexPlugin createPlugin(Executor pluginExecutor,
+	SimplexPlugin createPlugin(Executor pluginExecutor, Clock clock,
 			SimplexPluginCallback callback);
 }

api/net/sf/briar/clock/Clock.java

@@ -0,0 +1,11 @@
+package net.sf.briar.clock;
+
+/**
+ * An interface for time-related system functions that allows them to be
+ * replaced for testing.
+ */
+public interface Clock {
+
+	/** @see {@link java.lang.System#currentTimeMillis()} */
+	long currentTimeMillis();
+}

api/net/sf/briar/clock/SystemClock.java

@@ -0,0 +1,9 @@
+package net.sf.briar.clock;
+
+/** Default clock implementation. */
+public class SystemClock implements Clock {
+
+	public long currentTimeMillis() {
+		return System.currentTimeMillis();
+	}
+}