Is it ok to use a method to get a state *IF* I never, ever want to set that state?

class Book {
  constructor(title, author, isbn) {
    this.title = title;
    this.author = author;
    this.isbn = isbn;
    this.patron = null;
    this.dueDate = null;
    this._checkedOut = false;
  }

  static newDueDate() {
    const date = new Date();
    date.setDate(date.getDate() + 14);
    return date;
  }

  set checkedOut(bool) {
    this._checkedOut = bool;

    if (bool) {
      this.dueDate = Book.newDueDate();
    } else {
      this.dueDate = null;
    }
  }

  get checkedOut() {
    return this._checkedOut;
  }

  isOverdue() {
    const now = new Date();
    if (this.dueDate && this.dueDate < now) {
      return true;
    } else {
      return false;
    }
  }

  daysOverdue() {
    const now = new Date();

    if (this.dueDate && this.dueDate < now) {
      const daysOverdue = Math.floor((now - this.dueDate) / 86400000);
      return daysOverdue;
    } else {
      return 0;
    }
  }
}

chargeFines() {
    this.patrons.forEach((patron) => {
      let book = patron.currentBook;
      if (book.isOverdue()) {
        patron.balance +=
          Math.ceil(this.dailyFine * book.daysOverdue() * 100) / 100;
        book.dueDate = new Date();
      }
    });