JavaScript Date Object

The JavaScript Date object enables you to work with, and manipluate time (and no, it doesn't enable you to travel back through time to buy a lottery ticket with this week's winning numbers - if it did that I would have already done it, and would be sitting on beach somewhere instead of writing this book).

The Date object enables the JavaScript develper to create a Date object initialized to a specific time and date, or to create a date object that reflects the current system time on the computer on which the browser is running. There are two important things to note when working with the JavaScript Date object. Firstly, when reading the date and time of the user's computer you are completely at the mercy of the user's ability to set and maintain the correct day and time on their computer. Secondly, whilst you can read the system date and time set on the user's computer, and change those settings within your Date object instance, you cannot change the computer's system date and time. No matter what methods you call or properties you change on your Date object, the user's system date and time remain unaltered.

Understanding System Time

Rather than understanding the concepts of dates and times, computers essentially record the passage of time since a particular baseline date and time (often referred to as the epoch). The epoch date, and unit of measurement for elapsed time depends on the system being used. A Windows system, for example, counts the number of 100-nanoseconds since January 1, 1601 00:00:00. A UNIX or LINUX based system, on the other hand, counts the number of seconds that have elapsed since January 1, 1970 00:00:00.

When working with JavaScript it is useful to know that when the current date is requested it is returned as the number of milliseconds that have elapsed since January 1, 1970, 00:00:00. As you will see later in this chapter, the JavaScript Date object contains plenty of methods that make it easy for developers to set and read the time using human readable date formats.

Creating a JavaScript Date Object

A javaScript Date Object is created much like any other object in JavaScript, using the new keyword and some optional arguments (for an overview of Objects in JavaScript see JavaScript Object Basics). The syntax for creating an instance of a Date object is as follows:

var myDate = new Date(Optional Parameters);

The following table shows the different parameter options that are accpted by the Date object at Create time:

Parameters	Description	Example
None	Creates a date object set to the users current system time	var currentDate = new Date()
month dd, yyyy hh:mm:ss'	Sets Date object to specified month, day and time. Omitted values are set to zero	var myDate = new Date("December 1, 2009, 12:01:00")
mm, dd, yyyy	Initializes date object to the specified month, day and year.	var myDate = new Date(07, 24, 2008)
yyyy, mm, dd	Initializes the date object to the specified month, day and year	var myDate = new Date (10, 27, 2007, 10, 20)

Setting Specific Properties of a Date Object Instance

Once an instance of a date object has been created it is often necessary to change specific properties (such as the time or the year) of that object. As one might expect, JavaScript includes a number of methods in the Date object to facilitate this:

td>setMonth()

Method	Description	Example
setDate()	Sets the day of the month	myDate.setDate(20)
Sets the month of the year. Note that that this is a zero based value (i.e 0 = January)	myDate.setMonth(10)