I had few old Android devices lying around and needed digital clock. I could not find app with desired requirements, so I decided to create my own. No built-in advertisement. No in-app purchases. Requires Android 5.0 and up (2014+).
Screen TOUCH — Fullscreen ON /OFF.
Screen SWIPE — Switch between 2 themes. One theme looks better on small phone screen, another on larger tablet screen.
There are no other settings. App shows your device's date and time:
Year - Month - Day · Day of the week (week starts on Monday) · Second
Hour (24h format) : Minute