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...
uptime
Show Date/Time Of Last Boot Get-WmiObject -class Win32_OperatingSystem | Select-Object __SERVER,@{label='LastBootUpTime';expression={$_.ConvertToDateTime($_.LastBootUpTime)}}