When they started in 1950 the MSF signals carried only seconds and minute markers, without any labels. In September 1974 a short burst of data at 100 bit/s was added during the first second of the minute to signal the hour and minute.

In November 1976 this was extended to give day and month, and to indicate whether summer time was in force.

In June 1977 another code at 1 bit/s was introduced by selectively lengthening the seconds pulses. This was then called the 'slow code' and the earlier code the 'fast code'.

The current time and date code, which was previously known as the ‘slow code’ provides more information (including the year, and day-of-week) and it is more resistant to interference as it can be received with a narrower bandwidth.

All commercially-produced equipment, such as radio-controlled clocks, uses the slow code. The fast code was removed in October 1998, restoring the full 500 ms minute marker.