The descendant is the point of the ecliptic opposite the ascendant, 180 degrees away by definition. It shares the ascendant's dependence on time, longitude and latitude, its full circuit in one sidereal day, its average of one degree every 3 minutes 59 seconds, and its very uneven rate.
Because it is defined as the opposite point rather than computed separately, no house system disagrees about it. That is unusual: in a chart where the second, third, fifth, sixth, eighth, ninth, eleventh and twelfth cusps can lie many degrees apart between systems, and where whole-sign and equal houses part company with the quadrant systems even about the Midheaven, the descendant is fixed by the ascendant and nothing else.
There is one nicety worth stating because it is commonly glossed over. The descendant is a point on the ecliptic, and a planet is on the descendant when its ecliptic longitude matches. That is not the same as the planet actually setting. A body with ecliptic latitude, which means every body except the Sun, sets a little before or after the moment its longitude reaches the descendant, and for the Moon, whose latitude runs to about five degrees, or for a steeply inclined asteroid, the difference can amount to several minutes of clock time and occasionally more.
Longitude is the working coordinate of the tradition, and it is a projection, not a sighting.