There's no magic to it. The person that loads the music onto the system plays it through until the intro ends then enters the current playing time into the metadata for the track. Then when it plays out for real in addition to the normal track remaining counter, there's a shutup counter counting down the intro. There's also usually an outro one as well.

I remember a presenter on one of the tinpot local stations round here came a cropper when he thought he had 25 seconds to fill which turned out to be 5 seconds.