APT Darkness Clock (APT DC) is a free app developed by Distinct Solutions Ltd. It is designed to help users calculate the optimal time for deep sky astrophotography or observing in a selected night and location. This app is a small subset of a full-featured desktop application called APT - Astro Photography Tool, which is like a Swiss army knife for your astro imaging sessions.

With APT DC, users can calculate the duration of DSD / NB Time and when these times start and end for a selected night and location. DSD Time refers to the darkest time of the night, between the evening astro twilight end, the morning astro twilight beginning, and when the Moon is below the horizon. On the other hand, NB Time refers to the time between astro twilights, which is important when imaging through narrow band filters.