C#: Finding Out Which Monday Is The 3rd Monday Of The Month?

Question

If I'm writing some C# code that runs through a year of dates (iterating by day) and want something special to happen every 3rd Monday of the month, how can I accomplish this?

In other words, what is the best way to find which Monday of the month a current Monday is?

I think your "other words" are saying something else. :) – MusiGenesis May 13 '10 at 14:47 — MusiGenesis, May 13 '10 at 14:47

score 13 · Accepted Answer · answered May 13 '10 at 14:50

13

public bool IsThirdMondayOfMonth(DateTime dt)
{
  if(dt.DayOfWeek == DayOfWeek.Monday && dt.Day > 14 && dt.Day <= 21)
  {
    return true;
  }
  return false;
}

answered May 13 '10 at 14:50

sechastain

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Better: `return (dt.DayOfWeek == DayOfWeek.Monday && dt.Day > 14 && dt.Day < 22);` – Josh Stodola May 13 '10 at 14:53
You might want this to be an extension for DateTime. You'll probably come across a number of things like this that are worth keeping in an extension library. – Carlos May 13 '10 at 14:55
Alrighty, I was hoping that there was something built in, but didn't think it was too likely. I guess I'll have to take some of these code snippets here. Thanks guys :) – sooprise May 13 '10 at 14:55

Adam Robinson · Answer 2 · 2010-05-13T14:58:03.063

I don't think your "in other words" really restates the problem that you describe first, so I'll answer both.

Here's a fairly simple method that will determine the nth occurrence of a particular day of the week in a given month in a given year.

public static DateTime DayOccurrence(int year, int month, DayOfWeek day, 
    int occurrenceNumber)
{
    DateTime start = new DateTime(year, month, 1);
    DateTime first = start.AddDays((7 - ((int)start.DayOfWeek - (int)day)) % 7);

    return first.AddDays(7 * (occurrenceNumber - 1));
}

Determining which Monday (or any other day) of the month a date is is much easier; just take the ceiling of the day of the month / 7:

public static int DayOccurrence(DateTime date)
{
    return (int)Math.Ceiling(date.Day / 7.0);
}

+1 Your edit was definitely worthwhile because I think that's what the OP is looking for — Josh Stodola, May 13 '10 at 15:02

score 1 · Answer 3 · answered May 13 '10 at 14:47

1

Find the Monday that is between the 15th and the 21st, inclusive.

answered May 13 '10 at 14:47

Toby

7,354
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score 0 · Answer 4 · answered May 13 '10 at 14:51

I don't know if there is a date manipulation library to do what you want, but you can write your own functions pretty easily:

using System;

class Program {
    static void Main(string[] args) {
        int year = 2010;
        int month = 05;
        DateTime thirdMonday = ThirdMonday(year, month);
    }

    private static DateTime ThirdMonday(int year, int month) {
        for (int day = 1; day <= DateTime.DaysInMonth(year, month); ++day) {
            DateTime dt = new DateTime(year, month, day);
            if (dt.DayOfWeek == DayOfWeek.Monday) {
                return dt.AddDays(14);
            }
        }
        // this should never happen
        throw new Exception();
    }
}

C#: Finding Out Which Monday Is The 3rd Monday Of The Month?

4 Answers4