Add Business Days and GetBusinessDays

Question

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

Answer 1

The only real solution is to have those calls access a database table that defines the calendar for your business. You could code it for a Monday to Friday workweek without too much difficult but handling holidays would be a challenge.

Edited to add non-elegant and non-tested partial solution:

public static DateTime AddBusinessDays(this DateTime date, int days)
{
    for (int index = 0; index <= days; index++;)
    {
        switch (date.DayOfWeek)
        {
            case DayOfWeek.Friday:
                date.AddDays(3);
                break;
            case DayOfWeek.Saturday:
                date.AddDays(2);
                break;
            default:
                date.AddDays(1);
                break;
         }
    }
    return date;
}

Also I violated the no loops requirement.

Answer 2

With internationalization, this is difficult. As mentioned in other threads here on SOF, holidays differ from country to country certainly and even from province to province. Most governments do not schedule out their holidays more than five years or do.

Answer 3

Latest attempt for your first function:

public static DateTime AddBusinessDays(DateTime date, int 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;
    }
}

Answer 4

public static DateTime AddBusinessDays(this DateTime date, int days)
{
    date = date.AddDays((days / 5) * 7);

    int remainder = days % 5;

    switch (date.DayOfWeek)
    {
        case DayOfWeek.Tuesday:
            if (remainder > 3) date = date.AddDays(2);
            break;
        case DayOfWeek.Wednesday:
            if (remainder > 2) date = date.AddDays(2);
            break;
        case DayOfWeek.Thursday:
            if (remainder > 1) date = date.AddDays(2);
            break;
        case DayOfWeek.Friday:
            if (remainder > 0) date = date.AddDays(2);
            break;
        case DayOfWeek.Saturday:
            if (days > 0) date = date.AddDays((remainder == 0) ? 2 : 1);
            break;
        case DayOfWeek.Sunday:
            if (days > 0) date = date.AddDays((remainder == 0) ? 1 : 0);
            break;
        default:  // monday
            break;
    }

    return date.AddDays(remainder);
}

Answer 5

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 = Convert.ToInt32(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);
}

Answer 6

I know I'm late to the game here, but here's yet another take on the AddBusinessDays method. Works with negative values, too. w00t!

public static DateTime AddBusinessDays(this DateTime dateTime, int businessDays)
{
 if (businessDays == 0) return dateTime;

 int weeks = businessDays / 5;
 int totalDays = (weeks * 7) + (businessDays % 5);

 DateTime result = dateTime.AddDays(totalDays);

 bool isNegative = businessDays < 0;
 if (isNegative)
 {
  // If we're subtracting days and the result falls on a day that is later in the
  // week than the day we started on, then we crossed over a weekend and need to
  // subtract two more days.
  if (result.DayOfWeek > dateTime.DayOfWeek)
  {
   result = result.AddDays(-2);
  }
 }

 // Handle the case where the result falls on a weekend.
 if (result.DayOfWeek == DayOfWeek.Saturday)
 {
  if (isNegative)
   result = result.AddDays(-1);
  else
   result = result.AddDays(2);
 }
 else if (result.DayOfWeek == DayOfWeek.Sunday)
 {
  if (isNegative)
   result = result.AddDays(-2);
  else
   result = result.AddDays(1);
 }

 return result;
}

Answer 7

using Fluent DateTime http://fluentdatetime.codeplex.com/

        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);
    }

Answer 8

Here is solution of ultimate elegance:

http://alecpojidaev.wordpress.com/2009/10/29/work-days-calculation-with-c/