Dates.day and Timestamp.day's return type should return DateType

api/src/main/java/org/apache/iceberg/transforms/Dates.java

@@ -61,6 +61,9 @@ public boolean canTransform(Type type) {
 
   @Override
   public Type getResultType(Type sourceType) {
+    if (granularity == ChronoUnit.DAYS) {
+      return Types.DateType.get();
+    }
     return Types.IntegerType.get();
   }
 

api/src/main/java/org/apache/iceberg/transforms/Timestamps.java

@@ -64,6 +64,9 @@ public boolean canTransform(Type type) {
 
   @Override
   public Type getResultType(Type sourceType) {
+    if (granularity == ChronoUnit.DAYS) {
+      return Types.DateType.get();
+    }
     return Types.IntegerType.get();
   }
 

api/src/test/java/org/apache/iceberg/transforms/TestDates.java

@@ -20,6 +20,7 @@
 package org.apache.iceberg.transforms;
 
 import org.apache.iceberg.expressions.Literal;
+import org.apache.iceberg.types.Type;
 import org.apache.iceberg.types.Types;
 import org.junit.Assert;
 import org.junit.Test;
@@ -53,4 +54,21 @@ public void testNullHumanString() {
     Assert.assertEquals("Should produce \"null\" for null",
         "null", Transforms.day(type).toHumanString(null));
   }
+
+  @Test
+  public void testDatesReturnType() {
+    Types.DateType type = Types.DateType.get();
+
+    Transform<Integer, Integer> year = Transforms.year(type);
+    Type yearResultType = year.getResultType(type);
+    Assert.assertEquals(Types.IntegerType.get(), yearResultType);
+
+    Transform<Integer, Integer> month = Transforms.month(type);
+    Type monthResultType = month.getResultType(type);
+    Assert.assertEquals(Types.IntegerType.get(), monthResultType);
+
+    Transform<Integer, Integer> day = Transforms.day(type);
+    Type dayResultType = day.getResultType(type);
+    Assert.assertEquals(Types.DateType.get(), dayResultType);
+  }
 }

api/src/test/java/org/apache/iceberg/transforms/TestTimestamps.java

@@ -20,6 +20,7 @@
 package org.apache.iceberg.transforms;
 
 import org.apache.iceberg.expressions.Literal;
+import org.apache.iceberg.types.Type;
 import org.apache.iceberg.types.Types;
 import org.junit.Assert;
 import org.junit.Test;
@@ -82,4 +83,25 @@ public void testNullHumanString() {
     Assert.assertEquals("Should produce \"null\" for null",
         "null", Transforms.hour(type).toHumanString(null));
   }
+
+  @Test
+  public void testTimestampsReturnType() {
+    Types.TimestampType type = Types.TimestampType.withZone();
+
+    Transform<Integer, Integer> year = Transforms.year(type);
+    Type yearResultType = year.getResultType(type);
+    Assert.assertEquals(Types.IntegerType.get(), yearResultType);
+
+    Transform<Integer, Integer> month = Transforms.month(type);
+    Type monthResultType = month.getResultType(type);
+    Assert.assertEquals(Types.IntegerType.get(), monthResultType);
+
+    Transform<Integer, Integer> day = Transforms.day(type);
+    Type dayResultType = day.getResultType(type);
+    Assert.assertEquals(Types.DateType.get(), dayResultType);
+
+    Transform<Integer, Integer> hour = Transforms.hour(type);
+    Type hourResultType = hour.getResultType(type);
+    Assert.assertEquals(Types.IntegerType.get(), hourResultType);
+  }
 }