The Boston Navy Yard's radio station -- which at this time was assigned the callsign of "PG" -- had a spark transmitter, which meant it could only transmit the dots-and-dashes of Morse code. However, this wasn't a problem for broadcasting time signals, since these could be sent out as a standardized sequence of dots, with a long dash transmitted on the hour.
Electrician and Mechanic, March, 1909, page 403:
            This  department  is  devoted  to  the  Club  members  and  those  interested  in  Wireless  Telegraphy.    We  will     
     publish  experiences,  discoveries,  and  suggestions,  which  may  be  helpful  to  all  interested.

BOSTON,  MASS.,  January 24, 1909.      

    Sirs, -- In reply to Sampson Publishing Co. letter, regarding noon-time signals, as sent by wireless from this station, I submit the following report:--
    The time as sent by wireless is sent by the U. S. Naval Observatory, Washington, D. C., and is Washington time. It is sent over the telegraph wires, and through the medium of a sounder, acting as a transmitting key, operates the wireless transmitter.
    The signals commence at 11.55 A.M. and are sent out in dots, one for each second, except the 29th, 56th, 57th, 58th, 59th, and 60th seconds, which are omitted for the first four minutes; on the fifth and last minute, the 29th, 50th, 51st, 52d, 53d, 54th, 55th, 56th, 57th, 58th, 59th seconds are omitted, and on the 60th second a long dash is made, which signifies twelve o'clock noon.
    The signals are sent every day, except Sundays and holidays; it sometimes happens that owing to some fault on the telegraph lines, the signals are not sent until the first, second, third, and sometimes the fourth minute has gone by, and occasionally not received at all, over the telegraph wires.
Equipment Officer,
    Navy Yard,
        Boston, Mass.
Very respectfully,
    W.V. Albert, Ch. Elect. U.S.N.
        In charge of station.