JavaScript® Bible, Seventh Edition

Chapter 17. The Date Object

IN THIS CHAPTER

• Working with date and time values in JavaScript

• Performing date calculations

• Validating date entry form fields

Perhaps the most untapped power of JavaScript is its date and time handling. Scripters passed over the Date object with good cause in the early days of JavaScript, because in earlier versions of scriptable browsers, significant bugs and platform-specific anomalies made date and time programming hazardous without significant testing. Even with the improved bug situation, working with dates requires a working knowledge of the world's time zones and their relationships with the standard reference point, known as Greenwich Mean Time (GMT) or Coordinated Universal Time (abbreviated UTC).

Now that date- and time-handling has stabilized in modern browsers, I hope more scripters look into incorporating these kinds of calculations into their pages. In Chapter 57, "Application: Intelligent 'Updated' Flags," on the CD-ROM, for example, I show you an application that lets your web site highlight the areas that have been updated since each visitor's last surf ride through your pages—an application that relies heavily on date arithmetic and time-zone conversion.

Before getting to the JavaScript part of date discussions, however, the chapter summarizes key facts about time zones and their impact on scripting date and time on a browser. If you're not sure what GMT and UTC mean, the following section is for you.

Time Zones and GMT

By international agreement, ...