@Override
  public String getDisplayString(String[] children) {
    return getStandardDisplayString("date_add", children);
  }
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