Grebulon's Big Clock Screen Saver app turns your old useless device into a large full screen digital clock when the phone is sleeping.
The clock can show the time in various forms (24h, with or without seconds, etc.), fonts and colors. It can optionally also show the date (again in various forms). The clock is stationary - it doesn't move.

To set-up the screen saver, open the app and click on the settings button. It will take you directly to the phone's screen saver settings. Alternatively open Android's settings -> Display -> Screen saver.
Select "Grebulon Clock" and click on the gears icon to customize it. You can change the color, the time and date format (date can be hidden) and opt to show the time with or without seconds.

The clock will start depending on the screen saver settings. Usually it's set to start when the phone is charging.

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