Quality Center does not have a feature built-in to the UI for scheduling tests to run at a specific time as a background task. At least, versions 10 and 11 don't have that feature. I've heard that ALM v11.5 will have that feature, and people have told me older versions than 10 (but I don't know exactly which ones) had that feature.
With v10 and v11, the only way to schedule tests to run as at a specific time is to write a program to kick off the tests through the OTA API, and then use a third party scheduling tool (like MS Task Scheduler) to schedule that program.
There is an example of that code in the OTA API documentation if you look at the TSScheduler object.