Recently I wanted to cut a video (just export few seconds of a long video).
When everything still was AVI it was easy for me and I used VirtualDub. There I could decide if I just do a keyframe cut without having to encode it again, or a frame cut with having to encode this.
Nowadays having MKV, MP4 etc. I tried Lossless Cut which just gave me a black video. Open Shots Video Editor which is too complicated for this task and where I struggle to maintain the aspect ratio of the original video. Handbrake is a bit better, but also offers too many options for that purpose.
So I ended up using Online Video Cutter. But I wonder that there isn’t any open source that just works as easily as that.
I’m going to second the ffmpeg answers, they do the job but won’t be completely accurate to timestamps due to cutting at key frames. That’s how you persevere original quality though, and it’s fastest.
If you need to reencode to fit a target size as well, there exists tools for that too. I made my own for cutting shadowplay recordings by selecting start and stop points in the video and encoding them to fit the discord 8MB limit back in the day. My version is not exactly user friendly to get going though, and not updated for the last 5 years or so. Should still work though.