↑
Main Page

ECMA-262

This line creates a new

Date

object with the current date and time. You can also set the date and time

value when creating a new

Date

object in one of two ways. The first is to just specify the number of

milliseconds since 12 AM on January 1, 1970:

var d = new Date(0);

Two class methods (which would be static methods in Java) called

parse()

and

UTC()

can also be used

in conjunction with this method of creating

Date

objects. The

parse()

method accepts a string as an

argument and tries to convert that string into a date value (meaning the millisecond representation).

ECMA-262 doesn’t define the date formats that the

parse()

method accepts, so this is purely imple-

mentation-specific and often locale-specific. For instance, in the United States, most implementations

support the following date formats:

mm/dd/yyyy (such as 6/13/2004)

mmmm dd, yyyy (such as January 12, 2004)

For instance, if you wanted to create a

Date

object for May 25, 2004, you could use the

parse()

method

to get the millisecond representation and then pass that value into the

Date

constructor:

var d = new Date(Date.parse(“May 25, 2004”));

If the string passed in to

parse()

can’t be turned into a date, the function returns

NaN

The

UTC()

method also returns the millisecond representation of a date, but with different arguments:

year, month, day of the month, hours, minutes, seconds, and milliseconds. When using this method,

you must always specify the year and month, but the other information is optional. Be very careful

when setting the month because the values go from 0 to 11, where 0 is equal to January and 11 is equal

to December, so to set a date equal to February 5, 2004, you do this:

var d = new Date(Date.UTC(2003, 1, 5));

Here, the

represents February, the second month. This is obviously a very important difference to keep

track of when accepting user input to create a date. The other information is as you would expect, with

the possible exception that the hours are given in military time (0 through 23) instead of AM/PM. So, to

set a date equal to February 5, 2004 at 1:05 PM, you use this code:

var d = new Date(Date.UTC(2003, 1, 5, 13, 5));

The second method of creating a date is to specify the same arguments that

UTC()

accepts directly:

var d = new Date(2003, 1, 5);

The arguments are specified in the same order, and they don’t all need to be present (except for the year

and month).

The

Date

class is one of the few that overrides

toString()

and

valueOf()

differently. The

valueOf()

method always return the millisecond representation of the date whereas the

toString()

method

returns a string in an implementation-specific, human-readable format. For this reason, it is impossible

to depend on the

toString()

method for any consistent behavior. As an example, in the United States,

Chapter 3

06_579088 ch03.qxd 3/28/05 11:36 AM Page 78