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

Calculate the difference in the number of days. The time part of the string
will be ignored. If dateString1 is earlier than dateString2, then the
result can be negative.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFDateDiff.java