Originally used by the military for rocket launches, SMPTE time code is a standardized way of referencing the 24-hour period. Based around the video frame, the Society of Motion Picture and Television Engineers identify frames and sub frames with a unique identifier that includes hours, minutes, seconds, frames and sub frames. This way if two or more devices can read SMPTE time code, they will all have a common positional reference and keep themselves synchronized (actually both units will commence at the same positional reference, but then need to run at the same speed to stay locked. This is the function of a synchronizer.) SMPTE time code has no relationship to real time at any given moment, other than both use a 24-hour cycle. So program material that starts at hour one does not need to wait until 1 o'clock to function, but rather if a synchronized device is attached to the original device, it will also commence playback from the same hour location, unless an offset has been set in the slave device. There are various sub sets of time code that have to do with frame rates being different in film and television, but they are all still referenced to the 24-hour period.
(See Black Burst)