DesktopX has a builtin script for that, just make an object with DXbuilder, when the properties window comes up goto States tab select text for away state, goto Summery tab and name it under ObjectID say UptimeText or what ever you like, then hit Apply. Now select add on the bottom under additional of the General tab. Then System Information, hit OK. Now highlight it and select configure, under info type select Uptime and hit OK, then Apply. You can now go back to states and make the font size and color any way you want, because it should be displayed on your screen, under Summary you can also position it or you hold down the ctrl key and drag it where you want it. Once your done you can also make it a widget that will run by itself without the DX running. Hope that helps, good luck.
Forgot to say HAPPY NEW YEAR, when you start getting into this DesktopX, you'll find some amazing stuff you can do, I have the LCARS 2.0 (the old blue one)Star Trek looking theme on my computer, highly customized to me, I have almost forgotten what windows looks like, everyone freaks when they see my puter, but it pegs the cool factor meter, once they understand how it works.