diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailFiveTimeTrackerState.java b/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailFiveTimeTrackerState.java
index 573bbe989..9d2a0ce28 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailFiveTimeTrackerState.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailFiveTimeTrackerState.java
@@ -28,7 +28,6 @@ import org.joda.time.DateTime;
 import org.joda.time.Days;
 import org.joda.time.LocalDate;
 import org.joda.time.ReadablePeriod;
-import org.joda.time.Weeks;
 import org.zkoss.ganttz.util.Interval;
 
 /**
@@ -121,26 +120,8 @@ public class DetailFiveTimeTrackerState extends TimeTrackerStateUsingJodaTime {
     }
 
     @Override
-    protected Interval calculateIntervalWithMinimum(Interval candidateInterval) {
-        Interval resultInterval;
-        LocalDate startDate = LocalDate.fromDateFields(candidateInterval.getStart());
-        LocalDate endDate = LocalDate.fromDateFields(candidateInterval.getFinish());
-        Days numberOfDays = Days.daysBetween(startDate.toDateTimeAtCurrentTime(),
-                endDate.toDateTimeAtCurrentTime());
-
-        if (numberOfDays.getDays() < this.NUMBER_OF_DAYS_MINIMUM) {
-            LocalDate endIntervalDate = LocalDate.fromDateFields(candidateInterval.
-                    getStart()).
-                    toDateMidnight().
-                    plusWeeks(this.NUMBER_OF_DAYS_MINIMUM).toLocalDate();
-            LocalDate roundedEndIntervalDate = roundToNextYear(endIntervalDate);
-            resultInterval = new Interval(candidateInterval.getStart(),
-                    roundedEndIntervalDate.toDateMidnight().toDate());
-        } else {
-            resultInterval = candidateInterval;
-        }
-
-        return resultInterval;
+    protected Days getMinimumPeriod() {
+        return Days.days(NUMBER_OF_DAYS_MINIMUM);
     }
 
 }
\ No newline at end of file
diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailFourTimeTrackerState.java b/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailFourTimeTrackerState.java
index c63727a9e..ba376d3ee 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailFourTimeTrackerState.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailFourTimeTrackerState.java
@@ -20,13 +20,12 @@
 
 package org.zkoss.ganttz.timetracker.zoom;
 
-import org.jfree.data.time.Month;
 import org.joda.time.DateTime;
+import org.joda.time.Days;
 import org.joda.time.LocalDate;
 import org.joda.time.Months;
 import org.joda.time.ReadablePeriod;
 import org.joda.time.Weeks;
-import org.zkoss.ganttz.util.Interval;
 
 /**
  * Zoom level for months and years and weeks in the second level
@@ -95,27 +94,8 @@ public class DetailFourTimeTrackerState extends TimeTrackerStateUsingJodaTime {
     }
 
     @Override
-    protected Interval calculateIntervalWithMinimum(Interval candidateInterval) {
-        Interval resultInterval;
-        LocalDate startDate = LocalDate.fromDateFields(candidateInterval.getStart());
-        LocalDate endDate = LocalDate.fromDateFields(candidateInterval.getFinish());
-        Weeks numberOfWeeks = Weeks.weeksBetween(startDate.toDateTimeAtCurrentTime(),
-                endDate.toDateTimeAtCurrentTime());
-
-        if (numberOfWeeks.getWeeks() < this.NUMBER_OF_WEEKS_MINIMUM) {
-            LocalDate endIntervalDate = LocalDate.fromDateFields(candidateInterval.
-                    getStart()).
-                    toDateMidnight().
-                    plusWeeks(this.NUMBER_OF_WEEKS_MINIMUM).toLocalDate();
-            LocalDate roundedEndIntervalDate = roundToNextYear(endIntervalDate);
-
-            resultInterval = new Interval(candidateInterval.getStart(),
-                    roundedEndIntervalDate.toDateMidnight().toDate());
-        } else {
-            resultInterval = candidateInterval;
-        }
-
-        return resultInterval;
+    protected Days getMinimumPeriod() {
+        return Days.days(7 * NUMBER_OF_WEEKS_MINIMUM);
     }
 
 }
diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailThreeTimeTrackerState.java b/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailThreeTimeTrackerState.java
index f3f38db11..da680ea80 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailThreeTimeTrackerState.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/DetailThreeTimeTrackerState.java
@@ -20,13 +20,11 @@
 
 package org.zkoss.ganttz.timetracker.zoom;
 
-import java.util.Date;
-
 import org.joda.time.DateTime;
+import org.joda.time.Days;
 import org.joda.time.LocalDate;
 import org.joda.time.Months;
 import org.joda.time.ReadablePeriod;
-import org.zkoss.ganttz.util.Interval;
 import org.zkoss.util.Locales;
 
 /**
@@ -111,26 +109,8 @@ public class DetailThreeTimeTrackerState extends TimeTrackerStateUsingJodaTime {
     }
 
     @Override
-    protected Interval calculateIntervalWithMinimum(Interval candidateInterval) {
-        Interval resultInterval;
-        LocalDate startDate = LocalDate.fromDateFields(candidateInterval.getStart());
-        LocalDate endDate = LocalDate.fromDateFields(candidateInterval.getFinish());
-        Months numberOfMonths =
-            Months.monthsBetween(startDate.toDateTimeAtCurrentTime(),
-                endDate.toDateTimeAtCurrentTime());
-
-        if (numberOfMonths.getMonths() < this.NUMBER_OF_MONTHS_MINIMUM) {
-            LocalDate endIntervalDate = LocalDate.fromDateFields(candidateInterval.
-                    getStart()).
-                    toDateMidnight().
-                    plusMonths(this.NUMBER_OF_MONTHS_MINIMUM).toLocalDate();
-            LocalDate roundedEndIntervalDate = roundToNextYear(endIntervalDate);
-            resultInterval = new Interval(candidateInterval.getStart(),
-                    roundedEndIntervalDate.toDateMidnight().toDate());
-        } else {
-            resultInterval = candidateInterval;
-        }
-
-        return resultInterval;
+    protected Days getMinimumPeriod() {
+        return Days.days(NUMBER_OF_MONTHS_MINIMUM * 31);
     }
+
 }
\ No newline at end of file
diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/TimeTrackerStateUsingJodaTime.java b/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/TimeTrackerStateUsingJodaTime.java
index 34538f8d1..54be52a59 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/TimeTrackerStateUsingJodaTime.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/timetracker/zoom/TimeTrackerStateUsingJodaTime.java
@@ -25,8 +25,8 @@ import java.util.Collection;
 import java.util.Date;
 import java.util.List;
 
-import org.jfree.data.time.Month;
 import org.joda.time.DateTime;
+import org.joda.time.Days;
 import org.joda.time.LocalDate;
 import org.joda.time.ReadablePeriod;
 import org.zkoss.ganttz.util.Interval;
@@ -88,7 +88,25 @@ public abstract class TimeTrackerStateUsingJodaTime extends TimeTrackerState {
 
     protected abstract LocalDate round(LocalDate date, boolean down);
 
-    protected abstract Interval calculateIntervalWithMinimum(Interval candidateInterval);
+    protected abstract Days getMinimumPeriod();
+
+    private Interval calculateIntervalWithMinimum(Interval interval) {
+        ReadablePeriod minimumPeriod = getMinimumPeriod();
+        Days intervalDays = asPeriod(interval);
+        if (intervalDays.compareTo(minimumPeriod) >= 0) {
+            return interval;
+        }
+        LocalDate newEnd = new LocalDate(interval.getStart())
+                .plus(minimumPeriod);
+        return new Interval(interval.getStart(), newEnd
+                .toDateTimeAtStartOfDay().toDate());
+    }
+
+    private Days asPeriod(Interval interval) {
+        DateTime start = new DateTime(interval.getStart());
+        DateTime finish = new DateTime(interval.getFinish());
+        return Days.daysBetween(start, finish);
+    }
 
     @Override
     public Interval getRealIntervalFor(Interval testInterval) {
@@ -103,10 +121,4 @@ public abstract class TimeTrackerStateUsingJodaTime extends TimeTrackerState {
                 .toDateTimeAtStartOfDay().toDate());
         return result;
     }
-
-    protected LocalDate roundToNextYear(LocalDate date) {
-        LocalDate dateNextYear = date.withYear(date.getYear()+1).
-            withMonthOfYear(Month.JANUARY).withDayOfMonth(1);
-        return dateNextYear;
-    }
 }