code: public DateColumn set(Selection rowSelection, LocalDate newValue) {
          int packed = PackedLocalDate.pack(newValue);
          for (int row : rowSelection) {
              set(row, packed);
          }
          return this;
      }
docs: Conditionally update this column, replacing current values with newValue for all rows where the current value
  matches the selection criteria
  <p>
  Example:
  myColumn.set(myColumn.valueIsMissing(), LocalDate.now()); // no more missing values