DateTime extension methods

DateTimeExtensions.cs

using System;
using System.Collections.Generic;

public static class DateTimeExtensions
{
    public static bool IsBusinessDay(this DateTime date)
    {
        return date.DayOfWeek != DayOfWeek.Saturday
            && date.DayOfWeek != DayOfWeek.Sunday;
    }

    public static int BusinessDaysUntil(this DateTime from, DateTime to)
    {
        if (to < from)
        {
            DateTime aux = from;
            from = to;
            to = aux;
        }

        int businessDays = (to - from).Days + 1,
            fullWeeksTotal = businessDays / 7,
            intervalFirstDay = from.DayOfWeek == DayOfWeek.Sunday ? 7 : (int)from.DayOfWeek,
            intervalLastDay = to.DayOfWeek == DayOfWeek.Sunday ? 7 : (int)to.DayOfWeek;

        if (businessDays > (fullWeeksTotal * 7))
        {
            if (intervalFirstDay > intervalLastDay)
            {
                intervalLastDay += 7;
            }

            if (intervalFirstDay <= 6)
            {
                if (intervalLastDay >= 7) //Saturday and sunday are in the remaining interval
                {
                    businessDays -= 2;
                }
                else if (intervalLastDay == 6) //Only saturday is in the remaining interval
                {
                    businessDays -= 1;
                }
            }
            else //Only sunday is in the remaining interval
            {
                businessDays -= 1;
            }
        }

        //Remove weekend days
        return (businessDays -= fullWeeksTotal * 2);
    }

    public static int BusinessDaysUntil(this DateTime from, DateTime to, IEnumerable<DateTime> holidays)
    {
        int businessDays = from.BusinessDaysUntil(to);

        if (holidays != null)
        {
            from = from.Date;
            to = to.Date;

            DateTime current;
            foreach (DateTime holiday in holidays)
            {
                current = holiday.Date;
                if (current.IsBusinessDay() && current >= from && current <= to)
                {
                    --businessDays;
                }
            }
        }

        return businessDays;
    }
}