Converting Dates to Strings

There are a variety of ways to convert Date objects to strings. If you need to create a date string of a custom format, the best way to do so is to read the individual components from the object and piece the string together manually. If you want to create a string in a standard format, Date provides three methods to do so. These methods are toString(), toUTCString(), and toGMTString(), and their use is illustrated in the next example. Note that toUTCString() and toGMTString() format the string according to Internet (GMT) standards, whereas toString() creates the string according to “local” time. The result is shown in Figure 7-2.

Figure 7-2: Conversion of a Date object to a string

var appointment = new Date("February 24, 1996 7:45");
document.write("toString():", appointment.toString());
document.write("<<br />>");
document.write("toUTCString():", appointment.toUTCString());
document.write("<<br />>");
document.write("toGMTString():", appointment.toGMTString());

Converting Strings to Dates

Because you can pass the Date() constructor a string, it seems reasonable to assume that JavaScript provides a mechanism to convert strings into Date objects. It does so through the class method Date.parse(), which returns an integer indicating the number of milliseconds between the epoch and its argument. Notice that this method is a property of the Date constructor, not of individual Date instances.

The parse() method is very flexible with regard to the dates it can convert to milliseconds (the complete details of the method are found in Appendix B). The string passed as its argument can, naturally, be a valid string of the form indicated in Table 7-1. Also recognized are standard time zones, time zone offsets from GMT and UTC, and the month/day/year triples formatted with - or / separators, as well as month and day abbreviations like “Dec” and “Tues.” For example,

// Set value = December 14, 1982
var myDay = "12/14/82";                 
// convert it to milliseconds
var converted = Date.parse(myDay);      
// create a new Date object
var myDate = new Date(converted);       
// output the date
alert(myDate);

creates myDate with the correct value shown here:

If you are not sure whether the particular string you wish to convert will be recognized by Date.parse(), you need to check the value it returns. If it cannot convert the given string to a date, the method returns NaN. For example, the invocation in this example,

var myDay = "Friday, 2002";
var invalid = Date.parse(myDay);

results in NaN because myDay does not contain enough information to resolve the date.

Limitations of Date Representations

The nuances of the Date object should not be underestimated. Recall that ECMA-262 is the standard governing core JavaScript language features. While most aspects of browser implementations adhere to the specification rigorously, deviation in Date object behavior is commonplace. For example, Date support in very old browsers, particularly Netscape 2, is atrocious. There are so many bugs that the programmer is advised to avoid all but the simplest date operations on this platform. Netscape 3 is better, but still has problems handling time zones correctly. At the very least, caution should be exercised when manipulating dates in these two versions of Netscape. Internet Explorer 3 does not allow dates prior to the epoch. However, Netscape 4+ and Internet Explorer 4+ can handle dates hundreds and thousands of years before or after the epoch, which should be enough to handle most tasks. Of course, using extreme dates such as prior to 1 A.D. or far in the future should be done with caution. Appendix B contains full details on the various Date methods and implementation issues.