DATEVALUE takes a text argument. TODAY() is a numeric argument, as is
any cell with a formula of =TODAY(). This is why you can't use
DATEVALUE(TODAY()) or DATEVALUE(A1) if the formula in A1 is =TODAY().

As for your IF statement, you can do something like this:

=IF(A1<B2,"B2 hasn't happened yet","B2 has happened")