Always on Display - AMOLED and Smartwatch is a free app for Android devices that will display digital and analog clocks on your phone screen.

The application has the option to enable or disable notifications. You can also choose one of the many available clock styles.

To display the time and date on your phone screen, you will have to enable the service on your device. The clock will be displayed on the screen even if the device is locked.

The best thing about this is it's completely free.