“Division of Time” arrives at National Sawdust as a world premiere, placing new classical work inside one of Brooklyn’s key rooms for contemporary performance. Composer Eric Nathan shares the program with Gandini Juggling and cellist Jeffrey Zeigler, folding instrumental writing, physical patterning, and theatrical timing into a piece aimed at young listeners as well as adults. In a city that often treats new music as a specialist corner, this kind of cross-discipline format reads like a practical lesson in how time is counted, kept, and felt. The premise is simple, and the rigor is the point.