AddBusinessDays and GetBusinessDays

I need to find 2 elegant complete implementations of

public static DateTime AddBusinessDays(this DateTime date, int days)
{
 // code here
}

and 

public static int GetBusinessDays(this DateTime start, DateTime end)
{
 // code here
}

O(1) preferable (no loops).

EDIT: By business days i mean working days (Monday, Tuesday, Wednesday, Thursday, Friday). No holidays, just weekends excluded.

I already have some ugly solutions that seem to work but i wonder if there are elegant ways to do this. Thanks


This is what i've written so far. It works in all cases and does negatives too. Still need a GetBusinessDays implementation

public static DateTime AddBusinessDays(this DateTime startDate,
                                         int businessDays)
{
    int direction = Math.Sign(businessDays);
    if(direction == 1)
    {
        if(startDate.DayOfWeek == DayOfWeek.Saturday)
        {
            startDate = startDate.AddDays(2);
            businessDays = businessDays - 1;
        }
        else if(startDate.DayOfWeek == DayOfWeek.Sunday)
        {
            startDate = startDate.AddDays(1);
            businessDays = businessDays - 1;
        }
    }
    else
    {
        if(startDate.DayOfWeek == DayOfWeek.Saturday)
        {
            startDate = startDate.AddDays(-1);
            businessDays = businessDays + 1;
        }
        else if(startDate.DayOfWeek == DayOfWeek.Sunday)
        {
            startDate = startDate.AddDays(-2);
            businessDays = businessDays + 1;
        }
    }

    int initialDayOfWeek = (int)startDate.DayOfWeek;

    int weeksBase = Math.Abs(businessDays / 5);
    int addDays = Math.Abs(businessDays % 5);

    if((direction == 1 && addDays + initialDayOfWeek > 5) ||
         (direction == -1 && addDays >= initialDayOfWeek))
    {
        addDays += 2;
    }

    int totalDays = (weeksBase * 7) + addDays;
    return startDate.AddDays(totalDays * direction);
}

Latest attempt for your first function:

public static DateTime AddBusinessDays(DateTime date, int days)
{
    if (days < 0)
    {
        throw new ArgumentException("days cannot be negative", "days");
    }

    if (days == 0) return date;

    if (date.DayOfWeek == DayOfWeek.Saturday)
    {
        date = date.AddDays(2);
        days -= 1;
    }
    else if (date.DayOfWeek == DayOfWeek.Sunday)
    {
        date = date.AddDays(1);
        days -= 1;
    }

    date = date.AddDays(days / 5 * 7);
    int extraDays = days % 5;

    if ((int)date.DayOfWeek + extraDays > 5)
    {
        extraDays += 2;
    }

    return date.AddDays(extraDays);

}

The second function, GetBusinessDays, can be implemented as follows:

public static int GetBusinessDays(DateTime start, DateTime end)
{
    if (start.DayOfWeek == DayOfWeek.Saturday)
    {
        start = start.AddDays(2);
    }
    else if (start.DayOfWeek == DayOfWeek.Sunday)
    {
        start = start.AddDays(1);
    }

    if (end.DayOfWeek == DayOfWeek.Saturday)
    {
        end = end.AddDays(-1);
    }
    else if (end.DayOfWeek == DayOfWeek.Sunday)
    {
        end = end.AddDays(-2);
    }

    int diff = (int)end.Subtract(start).TotalDays;

    int result = diff / 7 * 5 + diff % 7;

    if (end.DayOfWeek < start.DayOfWeek)
    {
        return result - 2;
    }
    else{
        return result;
    }
}

using Fluent DateTime:

var now = DateTime.Now;
var dateTime1 = now.AddBusinessDays(3);
var dateTime2 = now.SubtractBusinessDays(5);

internal code is as follows

    /// <summary>
    /// Adds the given number of business days to the <see cref="DateTime"/>.
    /// </summary>
    /// <param name="current">The date to be changed.</param>
    /// <param name="days">Number of business days to be added.</param>
    /// <returns>A <see cref="DateTime"/> increased by a given number of business days.</returns>
    public static DateTime AddBusinessDays(this DateTime current, int days)
    {
        var sign = Math.Sign(days);
        var unsignedDays = Math.Abs(days);
        for (var i = 0; i < unsignedDays; i++)
        {
            do
            {
                current = current.AddDays(sign);
            }
            while (current.DayOfWeek == DayOfWeek.Saturday ||
                current.DayOfWeek == DayOfWeek.Sunday);
        }
        return current;
    }

    /// <summary>
    /// Subtracts the given number of business days to the <see cref="DateTime"/>.
    /// </summary>
    /// <param name="current">The date to be changed.</param>
    /// <param name="days">Number of business days to be subtracted.</param>
    /// <returns>A <see cref="DateTime"/> increased by a given number of business days.</returns>
    public static DateTime SubtractBusinessDays(this DateTime current, int days)
    {
        return AddBusinessDays(current, -days);
    }

I created an extension that allows you to add or subtract business days. Use a negative number of businessDays to subtract. I think it's quite an elegant solution. It seems to work in all cases.

namespace Extensions.DateTime
{
    public static class BusinessDays
    {
        public static System.DateTime AddBusinessDays(this System.DateTime source, int businessDays)
        {
            var dayOfWeek = businessDays < 0
                                ? ((int)source.DayOfWeek - 12) % 7
                                : ((int)source.DayOfWeek + 6) % 7;

            switch (dayOfWeek)
            {
                case 6:
                    businessDays--;
                    break;
                case -6:
                    businessDays++;
                    break;
            }

            return source.AddDays(businessDays + ((businessDays + dayOfWeek) / 5) * 2);
        }
    }
}

Example:

using System;
using System.Windows.Forms;
using Extensions.DateTime;

namespace AddBusinessDaysTest
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            label1.Text = DateTime.Now.AddBusinessDays(5).ToString();
            label2.Text = DateTime.Now.AddBusinessDays(-36).ToString();
        }
    }
}
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