There is a handy and free tool to check your system's uptime called "Uptime". (Clever, no?) It can be downloaded at http://uptimeexe.codeplex.com.
While it is very easy to use (just navigate to it in a command window and type "uptime"), it doesn't give much of an option for formatting the output to include only the days. So, a simple workaround is to use some "replace" commands within PowerShell and only output the number of days. Here's how: