The United States Naval Observatory began the A.1 scale on 13 September 1956, using an Atomichron commercial atomic clock, followed by the NBS-A scale at the National Bureau of Standards, Boulder, Colorado on 9 October 1957.
ðə juːnˈaɪɾᵻd stˈeɪts nˈeɪvəl əbzˈɜːvətˌoːɹi bɪɡˈæn ðɪ ˈeɪ.wˈʌn skˈeɪl ˌɔn θˈɜːtiːn sɛptˈɛmbɚ nˈaɪntiːnhˈʌndɹɪd fˈɪfti sˈɪks, jˈuːzɪŋ ɐn ɐtˈɑːmɪkɹən kəmˈɜːʃəl ɐtˈɑːmɪk klˈɑːk, fˈɑːloʊd baɪ ðɪ ˌɛnbˌiːˈɛsɐ skˈeɪl æt ðə nˈæʃənəl bjˈʊɹɹoʊ ʌv stˈændɚdz, bˈoʊldɚ, kˌɑːlɚɹˈɑːdoʊ ˌɔn nˈaɪn ɑːktˈoʊbɚ nˈaɪntiːnhˈʌndɹɪd fˈɪfti sˈɛvən.