You must split the calculation into smaller pieces
instead of trying to stuff it into one single line.
This will make it clearer to understand.
The first step is to calculate the first day of a quarter like this:
var firstDayOfQuarter = new DateTime(year, 1 + (quarter - 1) * 3, 1);
Then you have to add 4 months and to subtract 1 day.
var dueDate = firstDayOfQuarter.AddMonths(4).AddDays(-1);
A method for calculation would look like this:
DateTime GetDueDate(int year, int quarter)
{
var firstDayOfQuarter = new DateTime(year, 1 + (quarter - 1) * 3, 1);
var dueDate = firstDayOfQuarter.AddMonths(4).AddDays(-1);
return dueDate;
}