AddWorkingDays C# With Holidays Support

DateMath.cs

using System;
using System.Collections.Generic;
using System.Text;

namespace ReportAutomation
{
    public static class DateMath
    {
        public static DateTime AddWorkingDays(this DateTime input, int interval, List<DateTime> holidays = null)
        {
            bool hasHolidays = (holidays != null);
            DateTime cursor = input;
            int nextHolidayIndex = 0;
            int daysNextHoliday = -1;
            int daysNextWeekend = -1;
            int nbdAdd = 0;
            int nbdAdd2 = 0;
            bool adjust = false;
            while (cursor.DayOfWeek == DayOfWeek.Saturday || cursor.DayOfWeek == DayOfWeek.Sunday || 
                (hasHolidays && holidays.FindIndex(item => item.Equals(cursor)) != -1))
            {
                cursor = cursor.AddDays(1);
                adjust = true;
            }
            if (adjust && interval > 0) interval--;
            if (hasHolidays) 
            {
                nextHolidayIndex = holidays.FindIndex(item => item >= cursor);
                if (nextHolidayIndex == -1)
                {
                    hasHolidays = false;
                }
                else
                {
                    daysNextHoliday = holidays[nextHolidayIndex].Subtract(cursor).Days;
                }
            }
            do {
                daysNextWeekend = (cursor.DayOfWeek == DayOfWeek.Sunday)?0:(DayOfWeek.Saturday - cursor.DayOfWeek);
                if (hasHolidays && daysNextHoliday < 0)
                {
                    nextHolidayIndex++;
                    if (nextHolidayIndex >= holidays.Count)
                    {
                        hasHolidays = false;
                    }
                    else
                    {
                        daysNextHoliday = holidays[nextHolidayIndex].Subtract(cursor).Days;
                    }
                }

                if (hasHolidays && (daysNextHoliday < daysNextWeekend))
                {
                    nbdAdd = daysNextHoliday;
                    nbdAdd2 = 1;
                }
                else
                {
                    nbdAdd = daysNextWeekend;
                    nbdAdd2 = 2;
                }
                if (nbdAdd <= interval)
                {
                    if (hasHolidays) daysNextHoliday -= nbdAdd + nbdAdd2;
                    cursor = cursor.AddDays(nbdAdd + nbdAdd2);
                    interval -= nbdAdd;
                }
                else
                {
                    cursor = cursor.AddDays(interval);
                    interval = 0;
                }
            } while (interval > 0 || (hasHolidays && daysNextHoliday == 0) || daysNextWeekend == 0);
            return cursor;
        }
    }
}

Works fast enough for my needs however would be great if it could be optimised.