There is intentionally no Timebutler app in the Google Play Store or Apple App Store - for many different reasons. Nevertheless, you can of course use Timebutler with any device, PC or laptop, tablet or smartphone, and all Timebutler features are available on all devices.
You do not need an app to use Timebutler on a smartphone or tablet, because:
Any user can simply open a browser on a smartphone or tablet and go to timebutler.de. Timebutler is optimized for all devices. When signing in, you can enable the “Stay signed in” option; then, on future visits, you will go straight to your Timebutler user account without entering a username and password.
You can also add the page as an icon on your smartphone. Then you will have an icon on your smartphone, tap it and you will go straight to your Timebutler user account - just like an app. When you tap the icon, you will immediately enter your Timebutler user account, you are signed in, and you can use all features without restriction.