I've seen a few webms that have very unusual durations, even in the negatives. I've been trying to recreate it with ffmpeg, but I don't know enough of that program to make it work, if it would work at all. Is there anything else I could use? Do I have to do something between converting it from mp4 to webm?
I figured it out. The way I did it, was I got MKVToolNix for Windows, then I opened the webm file in the file info tab, then I looked at the duration area. There's a location tab, and there's a number next to duration on that. If you change the values after that with a hex editor (I used Frhed) then it will break the video duration.