S. V. Gupta, Ch. 1.2.4 Time Measurements, in Robert Hall, Richard M. Jr. Osgood, Jurgen Parisi, Hans Warlimont (a cura di), Units of Measurement: Past, Present and Future. International System of Units, Springer Series in Materials Science: 122, Springer, 2010, pp. 7–8, ISBN9783642007378.