A free program for Android, by Fat Bytes.

Always-on Display – AMOLED is a powerful app that makes your device screen stay always on and allows you to use the screen for other things, like viewing your photos or videos, playing music, watching movies, and more. 

With this app, you can customize the always-on display screen to show the information you need. You can use it to view the time, date, and region. If you need to check for any information, you can set a reminder and get notified. It will also allow you to check the temperature and the weather.

You can also choose from different clock designs and change the time size and format.