sla calculator c#

public DateTime? CalculateSLADueDate(DateTime slaStartDateUTC, double slaDays, TimeSpan workdayStartUTC, TimeSpan workdayEndUTC)
{
  if ((slaDays < 0)
  || (workdayStartUTC > workdayEndUTC))
  {
    return null;
  }

  var dueDate = slaStartDateUTC;
  var tsWorkdayHours = (workdayEndUTC - workdayStartUTC);
  var tsSlaCount = TimeSpan.FromHours(slaDays * ((workdayEndUTC - workdayStartUTC)).TotalHours);

  //get list of public holidays from in-memory cache
  var blPublicHoliday = new PublicHoliday();
  IList publicHolidays = blPublicHoliday.SelectAll();

  do
  {
    if ((dueDate.DayOfWeek == DayOfWeek.Saturday)
    || (dueDate.DayOfWeek == DayOfWeek.Sunday)
    || publicHolidays.Any(x => x.HolidayDate == dueDate.Date)
    || ((dueDate.TimeOfDay >= workdayEndUTC) && (dueDate.TimeOfDay < workdayStartUTC)))
    {
      //jump to start of next day
      dueDate = dueDate.AddDays(1);
      dueDate = new DateTime(dueDate.Year, dueDate.Month, dueDate.Day, workdayStartUTC.Hours, workdayStartUTC.Minutes, workdayStartUTC.Seconds);
    }
    else if ((dueDate.TimeOfDay == workdayStartUTC) && (tsSlaCount >= tsWorkdayHours))
    {
      //add a whole working day
      dueDate = dueDate.AddDays(1);
      tsSlaCount = tsSlaCount.Subtract(tsWorkdayHours);
    }
    else if (dueDate.TimeOfDay == workdayStartUTC)
    {
      //end day - add remainder of time for final work day
      dueDate = dueDate.Add(tsSlaCount);
      tsSlaCount = tsSlaCount.Subtract(tsSlaCount);
    }
    else
    {
      if(workdayEndUTC > dueDate.TimeOfDay)
      {
        //start day and still in business hours - add rest of today
        tsSlaCount = tsSlaCount.Subtract(workdayEndUTC - dueDate.TimeOfDay);
        dueDate = dueDate.Add(workdayEndUTC - dueDate.TimeOfDay);
      }

      if (tsSlaCount.Ticks > 0)
      {
        //if theres more to process - jump to start of next day
        dueDate = dueDate.AddDays(1);
        dueDate = new DateTime(dueDate.Year, dueDate.Month, dueDate.Day, workdayStartUTC.Hours, workdayStartUTC.Minutes, workdayStartUTC.Seconds);
      }
    }
  }
  while (tsSlaCount.Ticks > 0);

  return dueDate;
}