@misc{1173526,
  author = {Katharine S Vandoren},
  title = {Letter Circular 600::timekeeping through the ages},
  year = {1940},
  month = {1940-01-01 05:01:00},
  publisher = {, National Institute of Standards and Technology, Gaithersburg, MD},
  doi = {https://doi.org/10.6028/NBS.LCIRC.600},
  language = {en},
}