How to Efficiently Calculate the Number of Business Days Between Two Dates in C#?

How to efficiently calculate the number of working days between two dates in C#

Calculating the number of working days between two dates takes into account weekends and possible holidays. C# provides several ways to accomplish this task.

A suggested solution utilizes the DateTime class to avoid inefficient day-by-day enumeration. This method calculates the number of working days in a specified interval by taking into account full weeks, partial weekends, and holidays. The algorithm is as follows:

1. Use the .Date attribute to convert the input date into the corresponding DateTime object.
2. Validate input by ensuring the first date is less than or equal to the second date.
3. Use TimeSpan to calculate the span between dates and increment the weekday count by 1 for each day.
4. Divide the number of working days by 7 to determine the number of complete weeks in the interval.
5. For remaining days that do not fit into a full week, check if they fall on a weekend (Saturday or Sunday). If so, reduce the working day count accordingly.
6. Subtract the number of weekends from the total number of working days during the full week.
7. Finally, iterate over any provided holidays and subtract them from the weekday count if they fall within the interval.

The following is the implementation of the solution:

public static int BusinessDaysUntil(this DateTime firstDay, DateTime lastDay, params DateTime[] bankHolidays)
{
    firstDay = firstDay.Date;
    lastDay = lastDay.Date;
    if (firstDay > lastDay)
        throw new ArgumentException("Incorrect last day " + lastDay);

    TimeSpan span = lastDay - firstDay;
    int businessDays = span.Days + 1;
    int fullWeekCount = businessDays / 7;

    // 计算周末天数
    int weekendDays = fullWeekCount * 2;
    int remainingDays = businessDays % 7;
    int firstDayOfWeek = (int)firstDay.DayOfWeek;
    int lastDayOfWeek = (int)lastDay.DayOfWeek;

    if (remainingDays > 0)
    {
        if (lastDayOfWeek == 0) lastDayOfWeek = 7; // 将周日转换为7
        if (firstDayOfWeek == 0) firstDayOfWeek = 7; // 将周日转换为7

        if (lastDayOfWeek < 6)
            weekendDays += 0; // 如果最后一天不是周末，则不加周末天数
        else if (lastDayOfWeek == 6)
            weekendDays += 1; // 如果最后一天是周六，则加一天
        else
            weekendDays += 2; // 如果最后一天是周日，则加两天

        if (firstDayOfWeek == 7)
            weekendDays -= 1; // 如果第一天是周日，则减一天
        else if (firstDayOfWeek == 6)
            weekendDays -= 1; // 如果第一天是周六，则减一天

    }

    businessDays -= weekendDays;

    foreach (DateTime bankHoliday in bankHolidays)
    {
        DateTime bh = bankHoliday.Date;
        if (bh >= firstDay && bh <= lastDay)
            businessDays--;
    }

    return businessDays;
}

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