I am going to have to assume that the second parameter is hidden, otherwise this would appear to be near impossible (or extremely complicated), since whatever logic you use will be based on the value in the 1st parameter.

For the 2nd parm, set your default value based on the other parm's value, like so:

=DateAdd(DateInterval.Day,-365,Parameters!StartDate.Value)

That's it. The other way this could be accomplished would be to use a global variable (the value of the 1st parm again), but this method for the scenario you described is much easier.