PDC is a signal intended for controlling VCRs. It is broadcast along with ordinary television as part of Teletext and it tells the VCR when a particular programme starts and ends. Thus a PDC-equipped VCR will always record the entirety of a programme, even if it is re-scheduled.
To learn more about PDC, follow one of these links and see:
Update your links! Since its inception in 1996, the PDC Live Home Page has used several different addresses to refer to the same site. Most of the addresses still work, but I should like to encourage maintainers of links to use only http://pdc.ro.nu/ . It's shorter than any of the others, and as it is part of my own http://ro.nu/ domain, I can guarantee that it will function for the indefinite future.