Question

public static DateTime GetBudgetYearEndDateConsideringLeapYear(DateTime budgetYearStartDate)
    {
        DateTime endDate = DateTime.MinValue;
        if (budgetYearStartDate == null)
            throw new ArgumentNullException("budgetYearStartDate must be provided before calling this method");

        int startingDateMonth = budgetYearStartDate.Month;
        if (startingDateMonth > 2) /// Leap year for the starting year is escaped because  Month is NOT February
        {
            endDate = budgetYearStartDate.AddDays(365);
            bool isEndYearLeapYear = DateTime.IsLeapYear(endDate.Year);

            if (isEndYearLeapYear)
            {
                return endDate.AddDays(1);
            }
            return endDate;
        }
        else  // started from JAN/ FEB
        {
            if (DateTime.IsLeapYear(budgetYearStartDate.Year))
            {
                return budgetYearStartDate.AddDays(366);
            }
            return budgetYearStartDate.AddDays(365);
        }

    }

What am I missing here

Answer 1

How about startDate.AddYears(1).AddDays(-1);?

This assumes that given a start date of July 1, you want an end date of June 30. The example you posted suggests that you want July 1 -> July 1, in which case, it would be startDate.AddYears(1);.