Nikon 1 J5 Reference Manual page 179

Delete the unwanted frames.
4
Press 1 to delete all frames before
(Choose start point) or after (Choose
end point) the current frame.
5 Save the copy.
Highlight Yes and press J to save the
edited copy. If necessary, the copy can
be trimmed as described above to
remove additional footage.
D
Trimming Movies
Movies must be at least two seconds long (or, in the case of 4K and
slow-motion movies, at least three seconds long). If a copy can not be
created at the current playback position, the current position will be
displayed in red in Step 4 and no copy will be created. The copy will not
be saved if there is insufficient space available on the memory card.
To prevent the camera from turning off unexpectedly, use a fully-
charged battery when editing movies.
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