The plugin fetches all releases the recording appears on and uses the earliest date of release and sets it in a custom tag called firstrecorded. Note: Other then suggested by the tag name this is the date of first release, not the date when it was recorded.
The functionality of getting this first release date for a recording has been integrated into MusicBrainz and Picard 2.6 or later provides the %_recording_firstreleasedate% to hold this date. If you want to reproduce the exact behavior of the plugin and have this in the firstrecorded tag you could use a script:
$set(firstrecorded,%_recording_firstreleasedate%)
If you actually want to have the date of recording have a look at the Recording date plugin instead.