They enable you to keep track of your busy schedule no matter where you are. With the right software or plugin you can even synchronize them with your main desktop calendar such as Microsoft Outlook or Apple iCal, or with your PDA.
It does only one thing, but it does it very efficiently and very well: it tells you whether today is Christmas or not.
My only criticism is that it doesn’t tell you in which time zone the site is hosted.
For example, San Francisco is 8 hours behind UK time, so if the site is hosted in London and some poor kid in SF might check out the site at 16:01 his time and spoil his whole Christmas experience by opening his presents a day early.
Maybe it should be called Is it Christmas (give or take 12 hours, depending upon in which time zone you live)?.