Trim MP4

Compressed video stores a complete picture - a keyframe - every few seconds, and the frames in between only describe what changed. A cut that copies the video without re-encoding can only start on a keyframe, because the frames after it make no sense without one. So Wave's fast cut starts at the last keyframe at or before your start time, which can be a second or two earlier than you typed (phone cameras typically store one every second or two; screen recorders and streaming captures can space them further apart). The end of the clip is accurate.

Exact cut decodes the video from the keyframe before your start, throws away the frames before the start time, and re-encodes the rest to H.264, so the clip begins on the exact frame. It takes longer - the clip is decoded and encoded in the browser - and costs a little detail, like any re-encode. When something else already forces a re-encode (a size target, a lower resolution, or Make it play everywhere), cuts are always exact.

Times go in as minutes and seconds, like 1:05, with tenths if you need them (0:12.5); hours work too (1:02:30). Leave Start empty to begin at the start of the file, or End empty to run to the end. The plan shows the kept range and the size estimate updates for the shorter clip. Trimming works for MP3 output as well, which is handy for cutting the audio of one song out of a long recording.

On the command line, ffmpeg does the fast cut with `ffmpeg -ss 1:30 -to 3:45 -i input.mp4 -c copy output.mp4` and the exact one with `ffmpeg -ss 1:30 -to 3:45 -i input.mp4 -c:v libx264 -crf 20 -c:a aac output.mp4`. Putting `-ss` before `-i` makes ffmpeg jump straight to the start instead of reading the whole file up to it. The difference between copying and re-encoding is covered in Remux vs re-encode.