>The first radio time signal, sent by telegraphic code, was broadcast in September 1903 by the United States Navy. Most sources list Navesink, New Jersey as the site of this
broadcast, but the reference clock was located at the United States Naval Observatory (USNO) in Washington, DC. The first regularly scheduled time broadcasts began on August 9, 1904* from the Navy Yard in Boston. By the end of 1905, the Navy had transmitted time signals from stations in several other cities, including Norfolk, Newport, Cape Cod, Key West, Portsmouth, and Mare Island in California
(PDF) https://tf.nist.gov/general/pdf/2131.pdf
*Another source said August 9, 1905