After performing a Screen Recording on a PP slide, a sound bar appears below the slide.

At the end of the sound bar there are two symbols: <| |> which when hovered over, show:
<| Move Back 0.25 seconds (Alt+Shift+Left)
|> Move Forward 0.25 Seconds (Alt+Shift+Right)

But by clicking on either symbol or using Alt+Shit+ R/L no change in the duration of the audio in the sound bar takes effect.

Is that what the symbols and instructions are for? To cut off the end or lengthen the beginning of the audio? How are those cutoffs executed, please?

My question is: How can I cut off the last second of a recording or shorten the delay at the beginning of the recording?

I know its an old post but in case someone needs the answer. Click on the sound object on your slide. A Playback ribbon tab will show up. Go to it and click on Trim Audio.

Thanks. But after trimming the audio to end within a fraction of a second, the final video still has the hanging silence.
The problem occurs on every video so that regardless of how finely trimmed the sound bars are, there's still hanging silence before the next section starts.

I see. that does make it difficult. My guess is that PowerPoint is not going to edit anything but 1 second increments. For this task you would be best to use Audacity or another DAW. Fix up the audio to have no silence then insert back into PowerPoint.