APT Darkness Clock (APT DC) is a mobile app based on Deep Sky Darkness calculator. It's primary purpose is showing the DSD Time duration, start and end. DSD means Deep Sky Darkness, the time suitable for imaging faint objects. With other words when there is no Moon above horizon and twilights are away. Even if you are using top class narrowband filters, there is need to know when the astro twilight ends/starts.
APT DC is a free app, without any kind of ads or in-app purchases. In case you like it, consider Rewarding our work.
Scan the QR code with your phone, or click it to navigate the corresponding app store.
Here are screenshots of the main screen.
There is a version for Google WearOS watches. It shows the essential information about the DSD and NB (NarrowBand time) as well as your location for easy checkup. Here is QR code to scan with your phone and install it on the watch.