I recently upgraded my Roku from an SX3 to a Roku Ultra. Setup went well with the exception that I used to have the digital time constantly on my screen when we were not using the Roku unit for viewing. Now that no longer works. I can get the digital time screensaver for 10 or 15 minutes before it disappears and it displays in the lower right corner of the screen rather than in the middle. We used the Roku digital time screensaver as a clock and I would like it back.
You may have "power saving" enabled under Settings->System->Power. It will put the Roku into a power saving mode after 20 minutes which will turn off the output from the Roku.