강의를 보며 참조하고 있는 책인데... 의문이 생겨서 질문드립니다.

아래 코드에서...
weekFromToday = new Date(today.getTime() + 7 * 24 * 60 * 60 * 1000);

weekFromToday의 Date객체는 getTime()이라는 메소드를 사용하여 밀리세컨드 값을 가지게 되는데....

그 아래 코드를 보면...

day = dayNames[weekFromToday.getDay()]; 이란 코드가 나옵니다.

weekFromToday 객체 값은 밀리세컨드인데 여기서 어떻게 getDay() 즉, 요일값(0~6)을 가지고 올 수 있는지 이해가 가질 않습니다.

답변 부탁드리겠습니다.

------------------------------------------------------------------------
/* The script is placed inside an immediately invoked function expression
which helps protect the scope of variables */

(function() {

// PART ONE: CREATE HOTEL OBJECT AND WRITE OUT THE OFFER DETAILS

// Create a hotel object using object literal syntax
var hotel = {
name: 'Park',
roomRate: 240, // Amount in dollars
discount: 15, // Percentage discount
offerPrice: function() {
var offerRate = this.roomRate * ((100 - this.discount) / 100);
return offerRate;
}
};

// Write out the hotel name, standard rate, and the special rate
var hotelName, roomRate, specialRate; // Declare variables

hotelName = document.getElementById('hotelName'); // Get elements
roomRate = document.getElementById('roomRate');
specialRate = document.getElementById('specialRate');

hotelName.textContent = hotel.name; // Write hotel name
roomRate.textContent = '$' + hotel.roomRate.toFixed(2); // Write room rate
specialRate.textContent = '$' + hotel.offerPrice(); // Write offer price


// PART TWO: CALCULATE AND WRITE OUT THE EXPIRY DETAILS FOR THE OFFER
var expiryMsg; // Message displayed to users
var today; // Today's date
var elEnds; // The element that shows the message about the offer ending

function offerExpires(today) {
// Declare variables within the function for local scope
var weekFromToday, day, date, month, year, dayNames, monthNames;

// Add 7 days time (added in milliseconds)
weekFromToday = new Date(today.getTime() + 7 * 24 * 60 * 60 * 1000);

// Create arrays to hold the names of days / months
dayNames = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
monthNames = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'];

// Collect the parts of the date to show on the page
day = dayNames[weekFromToday.getDay()];
date = weekFromToday.getDate();
month = monthNames[weekFromToday.getMonth()];
year = weekFromToday.getFullYear();

// Create the message
expiryMsg = 'Offer expires next ';
expiryMsg += day + '
(' + date + ' ' + month + ' ' + year + ')';
return expiryMsg;
}

today = new Date(); // Put today's date in variable
elEnds = document.getElementById('offerEnds'); // Get the offerEnds element
elEnds.innerHTML = offerExpires(today); // Add the expiry message

// Finish the immediately invoked function expression
}());