Don't think there's a built-in method, but isn't this pretty trivial.
Forgive me for being a n00b, but don't you just need to do:
//assumes August 1st is your business start of year
DateTime startOfNewYear = new DateTime(DateTime.Now.Year, 8,1);
DateTime startOfFirstQuarter = startOfNewYear;
DateTime startOfSecondQuarter = startOfNewYear.AddMonths(3);
DateTime startOfThirdQuarter = startOfNewYear.AddMonths(6);
DateTime startOfForthQuarter = startOfNewYear.AddMonths(9);
DateTime endOfFirstQuarter = startOfSecondQuarter.AddDays(-1);
DateTime endOfSecondQuarter = startOfThirdQuarter.AddDays(-1);
DateTime endOfThirdQuarter = startOfForthQuarter.AddDays(-1);
DateTime endOfForthQuarter = startOfNewYear.AddYears(1).AddDays(-1);
Karl
