Idea for what to do when you don't know the exact date
I've seen it suggested that if you know the month and year but don't know the exact date you should put the first of the month and add a comment after the setlist saying that the date is a guess.
Problem:
One problem with this is that people don't read. I've seen things posted here as the first of the month with a comment saying the real date is unknown --- and I've seen that WRONG date posted on other sites as gospel and it's very difficult to get people to believe that it's not the correct date. (I've seen it happen.) The wrong date is on the internet forever and even when you discover the correct date people will argue about it because they already think they have the correct date because they saw it reposted on some fansite.
Solution:
Modify the site so you enter the day as 0 (because you have to store something in the database), but on screen on the setlist page it'd be listed as "month/year (full date not known)". That way there's no ambiguity and no wrong dates are propagated.