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

Subtract 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-sub
itests/util/src/main/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFCustomDateSub.java