Calculate the number of weekdays between two dates in C#

Question

How can I get the number of weekdays between two given dates without just iterating through the dates between and counting the weekdays?

Seems fairly straightforward but I can't seem to find a conclusive correct answer that abides by the following:

1. The total should be inclusive, so GetNumberOfWeekdays(new DateTime(2009,11,30), new DateTime(2009,12,4)) should equal 5, that's Monday to Friday.
2. Should allow for leap days
3. does NOT just iterate through all the dates between whilst counting the weekdays.

I've found a similar question with an answer that comes close but is not correct

Answer 1

O(1) solution:

// Count days from d0 to d1 inclusive, excluding weekends public static int countWeekDays(DateTime d0, DateTime d1) {     int ndays = 1 + Convert.ToInt32((d1 - d0).TotalDays);     int nsaturdays = (ndays + Convert.ToInt32(d0.DayOfWeek)) / 7;     return ndays - 2 * nsaturdays            - (d0.DayOfWeek == DayOfWeek.Sunday ? 1 : 0)            + (d1.DayOfWeek == DayOfWeek.Saturday ? 1 : 0); } 

Examples for January 2014:

    January 2014 Su Mo Tu We Th Fr Sa           1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31  countWeekDays(new DateTime(2014, 1, 1), new DateTime(2014, 1, 1)); // 1 countWeekDays(new DateTime(2014, 1, 1), new DateTime(2014, 1, 2)); // 2 countWeekDays(new DateTime(2014, 1, 1), new DateTime(2014, 1, 3)); // 3 countWeekDays(new DateTime(2014, 1, 1), new DateTime(2014, 1, 4)); // 3 countWeekDays(new DateTime(2014, 1, 1), new DateTime(2014, 1, 5)); // 3 countWeekDays(new DateTime(2014, 1, 1), new DateTime(2014, 1, 6)); // 4 

N.B. The DateTime inputs should be at around the same time of the day. If you are creating DateTime objects based solely on year, month, and day as in the examples above, then you should be fine. As a counter example, 12:01am on Jan 1 to 11:59pm Jan 2 spans only 2 days, but the above function will count 3 if you use those times.

Answer 2

From this link:

    public static int Weekdays(DateTime dtmStart, DateTime dtmEnd)     {         // This function includes the start and end date in the count if they fall on a weekday         int dowStart = ((int)dtmStart.DayOfWeek == 0 ? 7 : (int)dtmStart.DayOfWeek);         int dowEnd = ((int)dtmEnd.DayOfWeek == 0 ? 7 : (int)dtmEnd.DayOfWeek);         TimeSpan tSpan = dtmEnd - dtmStart;         if (dowStart <= dowEnd)         {             return (((tSpan.Days / 7) * 5) + Math.Max((Math.Min((dowEnd + 1), 6) - dowStart), 0));         }         return (((tSpan.Days / 7) * 5) + Math.Min((dowEnd + 6) - Math.Min(dowStart, 6), 5));     }     [1]: http://www.eggheadcafe.com/community/aspnet/2/44982/how-to-calculate-num-of-w.aspx 

Tests (each test returns 5):

    int ndays = Weekdays(new DateTime(2009, 11, 30), new DateTime(2009, 12, 4));     System.Console.WriteLine(ndays);      // leap year test     ndays = Weekdays(new DateTime(2000, 2,27), new DateTime(2000, 3, 5));     System.Console.WriteLine(ndays);      // non leap year test     ndays = Weekdays(new DateTime(2007, 2, 25), new DateTime(2007, 3, 4));     System.Console.WriteLine(ndays);