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On a Humax box generally you can’t set a recording from the guide once it’s started. On some (eg the 1000/1010/1100S series) you can record the programme in it’s entirety (from the time shift buffer) by simply pressing the record button, provided the box was tuned to that channel when the programme started.
If your settings for start time and end time are set to auto in the settings, broadcasters who properly implement accurate recording control the start and end recording times. The box wakes up 15 minutes before to watch for the recording start message from the broadcaster. Works well for the main channels BBC, ITV etc.
You will get more accurate information if you post the specific model as the capablity varies. EG. A Foxsat-HDR cannot record from the timeshift buffer so instant recording of a programme already started doesn’t work.