Download "Avidemux" from here:

https://www.fosshub.com/Avidemux.html

Open the software, and drag your video file into the viewer window. Mark the frame the RETRY Cursor disapears with [A] and mark the frame where the screen is FULLY WHITE/BLACK and Red's sprite is gone with [B] See your time under "Selection"

Save your new video by going under FILE and then choosing SAVE, it should have it's own icon.

NOTE: This only applies to the Mine Cart IL at the time of this post, but it can be used for other speedrun games in a similar way.

WHEN YOU MARK A and B, the saved video will only be between those selections.