The D'ni day was divided into 5 equal segments of roughly 6 hours, 3 minutes each, making for a 30-hour, 14 minute day. The D'ni watch was created to reflect this. As there was often no indication as to what time of day it was in D'ni by visible light, depending on your location in D'ni (or one of the Ages), the D'ni created their watches to also indicate what portion of the day it was, with the night-time hours being shaded a darker color than the daytime hours. Each of the 5 segments of the watch was thus shaded according to the activity of the bio-luminescent algae in the Lake, which also operated on a 30-hour cycle, lightening and darkening over that time.
The watch pictured here specifically belonged to Aitrus, and is taken from his sketch of it on his map depicting the path from D'ni to the Surface. It was later given to Gehn, probably by Ti'ana, and he used it regularly, though the only real mentions of it are during his trip back to D'ni with his son, Atrus.