diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/common/partialtime/TimeQuantity.java b/navalplanner-business/src/main/java/org/navalplanner/business/common/partialtime/TimeQuantity.java
index 2e4c0bd89..3008e8943 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/common/partialtime/TimeQuantity.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/common/partialtime/TimeQuantity.java
@@ -17,7 +17,7 @@ public class TimeQuantity {
                 Granularity.class);
         for (Granularity granularity : Granularity.values()) {
             Integer acc = sumAll(granularity, values);
-            if (acc != null) {
+            if (acc != null && acc != 0) {
                 result.put(granularity, acc);
             }
         }
@@ -30,7 +30,7 @@ public class TimeQuantity {
         for (EnumMap<Granularity, Integer> enumMap : maps) {
             if (enumMap.containsKey(granularity)) {
                 Integer valueToAdd = enumMap.get(granularity);
-                result = result == null ? enumMap.get(granularity) : result
+                result = result == null ? valueToAdd : result
                         + valueToAdd;
             }
         }
@@ -46,7 +46,11 @@ public class TimeQuantity {
         if (result.containsKey(granularity)) {
             newQuantity += result.get(granularity);
         }
-        result.put(granularity, newQuantity);
+        if (newQuantity != 0) {
+            result.put(granularity, newQuantity);
+        } else {
+            result.remove(granularity);
+        }
         return result;
     }
 
@@ -60,6 +64,20 @@ public class TimeQuantity {
         this(new EnumMap<Granularity, Integer>(Granularity.class));
     }
 
+    @Override
+    public boolean equals(Object obj) {
+        if (obj instanceof TimeQuantity) {
+            TimeQuantity other = (TimeQuantity) obj;
+            return values.equals(other.values);
+        }
+        return false;
+    }
+
+    @Override
+    public int hashCode() {
+        return values.hashCode();
+    }
+
     private TimeQuantity(EnumMap<Granularity, Integer> enumMap) {
         Validate.notNull(enumMap);
         this.values = enumMap;
diff --git a/navalplanner-business/src/test/java/org/navalplanner/business/common/test/partialtime/TimeQuantityTest.java b/navalplanner-business/src/test/java/org/navalplanner/business/common/test/partialtime/TimeQuantityTest.java
index af25e2766..5f1cb6a8f 100644
--- a/navalplanner-business/src/test/java/org/navalplanner/business/common/test/partialtime/TimeQuantityTest.java
+++ b/navalplanner-business/src/test/java/org/navalplanner/business/common/test/partialtime/TimeQuantityTest.java
@@ -1,6 +1,8 @@
 package org.navalplanner.business.common.test.partialtime;
 
 import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.CoreMatchers.not;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotSame;
 import static org.junit.Assert.assertThat;
 
@@ -90,6 +92,27 @@ public class TimeQuantityTest {
         assertThat(quantity.valueFor(Granularity.YEAR), equalTo(0));
     }
 
+    @Test
+    public void twoTimeQuantitiesAreEqualsIfHaveTheSameValues() {
+        assertEquals(TimeQuantity.empty(), TimeQuantity.empty());
+        assertEquals(TimeQuantity.empty().plus(3, Granularity.HOUR),
+                TimeQuantity.empty().plus(3, Granularity.HOUR));
+        assertEquals(TimeQuantity.empty().plus(3, Granularity.HOUR),
+                TimeQuantity.empty().plus(2, Granularity.HOUR).plus(1,
+                        Granularity.HOUR));
+        assertThat(TimeQuantity.empty().plus(2, Granularity.DAY),
+                not(equalTo(TimeQuantity.empty().plus(1, Granularity.DAY))));
+    }
+
+    @Test
+    public void equalsWorksWellWithZeroValues() {
+        TimeQuantity quantity = TimeQuantity.empty().plus(2, Granularity.MONTH)
+                .plus(-2, Granularity.MONTH);
+        assertEquals(quantity.hashCode(), TimeQuantity.empty().hashCode());
+        assertEquals(quantity, TimeQuantity.empty());
+        assertEquals(quantity, TimeQuantity.empty().plus(0, Granularity.MONTH));
+    }
+
     @Test
     public void theGreatestGranularitySpecifiedCanBeKnown() {
         TimeQuantity timeQuantity = TimeQuantity.empty().plus(2,