"SopDI' mara, wa'maH chorghlogh Qoylu'pu'."
Translation:Mara will eat at eighteen hundred hours.
That's not my point. The point is that the course is requiring the idiomatic (something happens) at (some time). And that's fine, but this also more literally means when (something happens), it is (some time). This form, whatever the time happens to be, should also be accepted.
Military time in Klingon is explained in Conversational Klingon. Eighteen hundred hours is expressed as wa'maH chorgh vatlh rep. Elsewhere, we learned of a rarer, non-military way to tell time, which is the Qoylu'pu' form. Duolingo is accepting both military time (eighteen hundred hours) and non-military time (eighteen o'clock) answers, and that's fine. That wasn't my point here.