The metro start screen for Windows 8 is one of its core new features. Apparently people are yet to get used to it as they are comparing it with the traditional desktop and start up menu of former Windows OS. The clock is not present in the start screen which includes various items to helps you navigate the OS with a single click.
Microsoft seems to have forgotten to place the clock on the start screen as most people working with their devices like to check the time from the desktop. Maybe with subsequent updates the clock will be included in the Start screen but for now you can follow these steps to get your clock on your start screen.
1. Install the visual studio 11 express which will help in running the clock application. You can easily get this application for free download.
2. Go to the Windows store and install the Windows 8 clock application. This will automatically bring the clock application on your start screen.
Unfortunately this application is not yet complete; it is in its beta form. You will not be able to automatically see the date and time on top of your screen. You must open the icon for it to display then disappear.