...have created a new calendar in which each new 12-month period is identical to the one which came before, and remains that way from one year to the next in perpetuity.

This adjustment was necessary in order to deal with the same knotty problem that makes designing an effective and practical new calendar such a challenge: the fact that each Earth year is 365.2422 days long. Hanke and Henry deal with those extra “pieces” of days by dropping leap years entirely in favor of an extra week added at the end of December every five or six years.