Ibn Ezra on Numbers 3:15

מבן חדש ומעלה. עד שוב המחבר' או על קרוב מהמקום הראשון זה אל זה והמשכיל יבין: FROM A MONTH OLD AND UPWARD. Until the sun and moon are once again in conjunction,²⁷If the child was born on the New Moon (rosh chodesh) (Rabbi Joseph Tov Elem). or until the moon returns to its approximate place of departure²⁸If the child was born at any other time of the month. and its original position and its present place are near each other.²⁹For when a child is not born on a New Moon, it is very difficult to visually determine when he is exactly a month old. The intelligent person will understand this.³⁰That a complete lunar cycle has astrological significance.