@Override
  public ObjectInspector initialize(ObjectInspector[] arguments) throws UDFArgumentException {
    if (arguments.length != 2) {
      throw new UDFArgumentLengthException(
        "date_add() requires 2 argument, got " + arguments.length);
    }
    if (arguments[0].getCategory() != ObjectInspector.Category.PRIMITIVE) {
      throw new UDFArgumentTypeException(0,
        "Only primitive type arguments are accepted but "
        + arguments[0].getTypeName() + " is passed. as first arguments");
    }
    if (arguments[1].getCategory() != ObjectInspector.Category.PRIMITIVE) {
      throw new UDFArgumentTypeException(1,
        "Only primitive type arguments are accepted but "
        + arguments[1].getTypeName() + " is passed. as second arguments");
    }

    inputType1 = ((PrimitiveObjectInspector) arguments[0]).getPrimitiveCategory();
    ObjectInspector outputOI = PrimitiveObjectInspectorFactory.writableDateObjectInspector;
    switch (inputType1) {
    case STRING:
    case VARCHAR:
    case CHAR:
      inputType1 = PrimitiveCategory.STRING;
      dateConverter = ObjectInspectorConverters.getConverter(
        (PrimitiveObjectInspector) arguments[0],
        PrimitiveObjectInspectorFactory.writableStringObjectInspector);
      break;
    case TIMESTAMP:
      dateConverter = new TimestampConverter((PrimitiveObjectInspector) arguments[0],
        PrimitiveObjectInspectorFactory.writableTimestampObjectInspector);
      break;
    case DATE:
      dateConverter = ObjectInspectorConverters.getConverter(
        (PrimitiveObjectInspector) arguments[0],
        PrimitiveObjectInspectorFactory.writableDateObjectInspector);
      break;
    default:
      throw new UDFArgumentException(
        " DATE_ADD() only takes STRING/TIMESTAMP/DATEWRITABLE types as first argument, got "
        + inputType1);
    }

    inputType2 = ((PrimitiveObjectInspector) arguments[1]).getPrimitiveCategory();
    switch (inputType2) {
      case BYTE:
        daysConverter = ObjectInspectorConverters.getConverter(
            (PrimitiveObjectInspector) arguments[1],
            PrimitiveObjectInspectorFactory.writableByteObjectInspector);
        break;
      case SHORT:
        daysConverter = ObjectInspectorConverters.getConverter(
            (PrimitiveObjectInspector) arguments[1],
            PrimitiveObjectInspectorFactory.writableShortObjectInspector);
        break;
      case INT:
        daysConverter = ObjectInspectorConverters.getConverter(
            (PrimitiveObjectInspector) arguments[1],
            PrimitiveObjectInspectorFactory.writableIntObjectInspector);
        break;
      default:
        throw new UDFArgumentException(
            " DATE_ADD() only takes TINYINT/SMALLINT/INT types as second argument, got " + inputType2);
    }

    return outputOI;
  }
UDFDateAdd.

Add a number of days to the date. The time part of the string will be
ignored.

NOTE: This is a subset of what MySQL offers as:
http://dev.mysql.com/doc/refman
/5.1/en/date-and-time-functions.html#function_date-add
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFDateAdd.java