So you have “one second” of your layer and want the same duration but further along in the timeline?
I’m guessing you have a clock or something similar which you want to keep cutting to, but to keep in time. I’d use Hold keyframes on the layer’s opacity to turn it on and off and copy and paste those to the points where you want it visible.
That sounds like a good workaround as I can slide the keyframes along, preserving the duration of the trim. I’m surprised AE can’t do what I want – basically ‘pan behind’ in reverse – but I shall move on!