«A Revolution in Timekeeping, part 3». A Walk Through Time. NIST (National Inst. of Standards and Technology). Arxivat de l'original el 28 maig 2007. [Consulta: 28 maig 2014].
«A Revolution in Timekeeping, part 3». A Walk Through Time. NIST (National Inst. of Standards and Technology). Arxivat de l'original el 28 maig 2007. [Consulta: 28 maig 2014].