protected VectorExpressionDescriptor.ArgumentType getInputColumnType() {
    return VectorExpressionDescriptor.ArgumentType.DATE;
  }
This is a superclass for unary functions and expressions taking a single timestamp and returning
a timestamp, that operate directly on the input and set the output.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/FuncDateToDate.java