You can adjust some settings on timeanddate.com, in order to make some services easier to use. Your settings are remembered when you visit the site later. Currently it is possible to customize time/date formats (e.g. 12-hour am/pm or 24/military), your default time zone, calendar preferences, and make a Personal World Clock.
You will be able to use many of the customization options without registering with the web site. However, without registration, all your settings are stored on your computer, and there is a chance that they will be lost after a while. To prevent this from happening, it is possible to register, so that your settings are stored on timeanddate.com.