If you want to make a clean install of the new OS X Mavericks or even the older versions of Mac OS X, than I strongly suggest you to create a bootable OS X key or a drive.
If you are using removable disk or USB key then you should look at this lesson. I will show you how you can safely remove your device from computer without the fear of losing data.
You are preparing a world wide event in your city and you want to be sure that every person around the world will know the exact time in their city. You will find out how in this tutorial.