added minus(long) to TimePoint/AbstractTimePoint

java/com.sap.sailing.domain.common/src/com/sap/sailing/domain/common/TimePoint.java

@@ -10,6 +10,8 @@ public interface TimePoint extends Comparable<TimePoint>, Serializable {
     
     TimePoint plus(long milliseconds);
     
+    TimePoint minus(long milliseconds);
+    
     boolean after(TimePoint other);
     
     boolean before(TimePoint other);

java/com.sap.sailing.domain/src/com/sap/sailing/domain/base/impl/AbstractTimePoint.java

@@ -41,6 +41,11 @@ public abstract class AbstractTimePoint implements TimePoint {
     public TimePoint plus(long milliseconds) {
         return new MillisecondsTimePoint(asMillis()+milliseconds);
     }
+    
+    @Override
+    public TimePoint minus(long milliseconds) {
+        return plus(-milliseconds);
+    }
 
     @Override
     public boolean after(TimePoint other) {