For an exact date, input the four-digit year, two-digit month, and two-digit date; for example 1998-06-20. Possible matches will show up as you type, just click on the one you want.
Since the search box shows partial matches for you to pick from, you can also search for year-and-month (e.g. 1998-06 for June 1998), the same month for all years (e.g. -06- for June), and month-and-day (-06-20 for June 20th).
Additionally, you can bring up a list of concerts for an entire decade by typing just "199" for the whole of the 1990s.