The International Time Bureau (BIH) began a time scale, Tm or AM, in July 1955, using both local caesium clocks and comparisons to distant clocks using the phase of VLF radio signals.
ðɪ ˌɪntɚnˈæʃənəl tˈaɪm bjˈʊɹɹoʊ bˌiːˌaɪˈeɪtʃ bɪɡˈæn ɐ tˈaɪm skˈeɪl, tˌiːˈɛm ɔːɹ ɐm, ɪn dʒuːlˈaɪ nˈaɪntiːnhˈʌndɹɪd fˈɪfti fˈaɪv, jˈuːzɪŋ bˈoʊθ lˈoʊkəl sˈiːsiəm klˈɑːks ænd kəmpˈæɹɪsənz tə dˈɪstənt klˈɑːks jˈuːzɪŋ ðə fˈeɪz ʌv vˌiːˌɛlˈɛf ɹˈeɪdɪˌoʊ sˈɪɡnəlz.