Complex sentence: As a practical matter this means that one cannot get a highly accurate result by using "n" and adding the actual periapsis date for a given year, but one can get high accuracy by using the formulation in terms of "D".
Split into simpler sentences: As a practical matter this means that one cannot get a highly accurate result by using "n" and adding the actual periapsis date for a given year. However, it is possible to get high accuracy by using the formulation in terms of "D".