I use 'system' often. It's for what are called 'Standard Properties' in DOpus. and is what I use in ExifToolGUI. The 'sytem' part is definitely not the problem as I have several working commands that include it.
Releasedate is used here:
I use that for renaming and it works--but only if the filename is exactly that length/has that many digits and I haven't figured out how to modify that.
For MP4 files, Release date is displayed in the Movies section of the metadata pane.