the recording length is currently measured using setInterval(..., 1000) & incrementing a counter of seconds which is not guaranteed to be accurate.

https://github.com/zkoss/zkcml/blob/v8.6.0.1/zkmax/src/archive/web/js/zkmax/med/Camera.js#L252-L259

Using absolutes timestamps - Date.now() - at various lifecycle events will improve the results.
MediaRecorder provides events onstart, onpause, onresume, onstop for that.
https://developer.mozilla.org/en-US/docs/Web/API/MediaRecorder/MediaRecorder