If you don't already know (hopefully you know), then this is what you call a clock. It keeps track of the time, and this one keeps the date as well. The default will be AM/PM mode, but you can use a 24 hour version of the clock as well. The button shows what mode it is in. Click it to change the mode. After 6:00 PM (or 18:00), the screen will turn dark because that what the clock has set as night. At 7:00 AM, you will find that it is bright. This is because the clock thinks it is day (even if it isn't).
Thanks to powercon5 for the text engine.