The date you set for your here will be the date that Beat ID will use as the starting point for searching the social web for your material. Beat ID will track the social web for anything uploaded on and after that date that uses your beat/song.
For example, if you first published a track 1 year again, but you set your tracking date for today, Beat ID will not search for any of your uses before today.
You should set your date as the day you first published that beat/song on the web. This will guarantee Beat ID is tracking for the full lifespan of your beat/song.