APT Darkness Clock (APT DC) is a free app without ads, that calculates the suitable time for deep sky astrophotography or observing in selected night and location. It is a small sub-set of a full featured desktop application named APT - Astro Photography Tool. APT is like Swiss army knife for your astro imaging sessions. No matter what are imaging with - Canon EOS, Nikon, CCD or CMOS astro camera, APT has the right tool for planning, collimating, aligning, focusing, framing, plate-solving, controlling, imaging, synchronizing, scheduling, analyzing, monitoring and more. You can find additional information about APT at www.astrophotography.app. To image or observe a faint deep sky objects is needed to use the darkest time of the night. This is the time between the evening astro twilight end, the morning astro twilight beginning and when the Moon is below the horizon. In APT that time is named DSD Time - Deep Sky Darkness Time. If the imaging is through narrow band filters, the Moon is less significant factor and important is the time between the astro twilights. This time is named NB Time - Narrow Band Time. The purpose of APT DC is to calculate what is the DSD / NB Time duration and when these times starts/ends for selected night and location. It is possible to use the current location or one of three other stored observing sites. For suggestions and support related to APT DC, use the dedicated section of the APT Forum at - http://aptforum.com/phpbb/viewforum.php?f=26