diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/util/Interval.java b/ganttzk/src/main/java/org/zkoss/ganttz/util/Interval.java
index b01b69d51..902149bf4 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/util/Interval.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/util/Interval.java
@@ -23,8 +23,12 @@
  */
 package org.zkoss.ganttz.util;
 
+import java.util.Arrays;
+import java.util.Collections;
 import java.util.Date;
 
+import org.apache.commons.lang.Validate;
+
 public class Interval {
     private final Date start;
 
@@ -79,4 +83,11 @@ public class Interval {
     private boolean isIncluded(Date date) {
         return start.compareTo(date) <= 0 && finish.compareTo(date) >= 0;
     }
+
+    public Interval coalesce(Interval otherInterval) {
+        Validate.notNull(otherInterval);
+        return new Interval(Collections.min(Arrays.asList(start,
+                otherInterval.start)), Collections.max(Arrays.asList(finish,
+                otherInterval.finish)));
+    }
 }
\ No newline at end of file