Nikon D3300 - Manual (Page 187)

167 Recording and Viewing Movies
D Trimming Movies
Movies must be at least two seconds long. The copy will not be saved
if there is insufficient space available on the memory card.
Copies have the same time and date of creation as the original.
A Choosing the Role of the Current Frame
To make the frame displayed in Step 5 the new end point ( x) instead of
the new start point ( w) or vice versa, press the A (L) button.
A (L) button