Get the week number of a date in C#

Published 25 May 2009

NOTE: This post was written in 2009. Although the core logic has not changed since then, the implementation has. For the lastest implementation, check out my NExtra project on GitHub.

When working with the DateTime class, it is not that straightforward to calculate the week number for a certain date. This post will present an alternative to the standard Globalization approach.

Note that this approach only works if the first weekday is Monday, the first week of a year is the one that includes the first Thursday of that year, and the last week of a year is the one that precedes the first calendar week of the next year.

I found the original version of the function at Simen Sandelien’s website. The web site has disappeared since then, and since I have modified the code a lot, I chose to publish it.

/// <summary>Get the week number of a certain date, provided that
/// the first day of the week is Monday, the first week of a year
/// is the one that includes the first Thursday of that year and
/// the last week of a year is the one that immediately precedes
/// the first calendar week of the next year.
/// </summary>
/// <param name="date">Date of interest.</param>
/// <returns>The week number.</returns>
public static int GetWeekNumber(this DateTime date)
{
    //Constants
    const int JAN = 1;
    const int DEC = 12;
    const int LASTDAYOFDEC = 31;
    const int FIRSTDAYOFJAN = 1;
    const int THURSDAY = 4;
    bool thursdayFlag = false;

    //Get the day number since the beginning of the year
    int dayOfYear = date.DayOfYear;

    //Get the first and last weekday of the year
    int startWeekDay = (int)(new DateTime(date.Year, JAN, FIRSTDAYOFJAN)).DayOfWeek;
    int endWeekDay = (int)(new DateTime(date.Year, DEC, LASTDAYOFDEC)).DayOfWeek;

    //Compensate for using monday as the first day of the week
    if (startWeekDay == 0) {
        startWeekDay = 7;
    }
    if (endWeekDay == 0) {
        endWeekDay = 7;
    }

    //Calculate the number of days in the first week
    int daysInFirstWeek = 8 - (startWeekDay);

    //Year starting and ending on a thursday will have 53 weeks
    if (startWeekDay == THURSDAY || endWeekDay == THURSDAY) {
        thursdayFlag = true;
    }

    //We begin by calculating the number of FULL weeks between
    //the year start and our date. The number is rounded up so
    //the smallest possible value is 0.
    int fullWeeks = (int)Math.Ceiling((dayOfYear - (daysInFirstWeek)) / 7.0);
    int result = fullWeeks;

    //If the first week of the year has at least four days, the
    //actual week number for our date can be incremented by one.
    if (daysInFirstWeek >= THURSDAY) {
        result = result + 1;
    }

    //If the week number is larger than 52 (and the year doesn't
    //start or end on a thursday), the correct week number is 1.
    if (result > 52 && !thursdayFlag) {
        result = 1;
    }

    //If the week number is still 0, it means that we are trying
    //to evaluate the week number for a week that belongs to the
    //previous year (since it has 3 days or less in this year).
    //We therefore execute this function recursively, using the
    //last day of the previous year.
    if (result == 0) {
        result = GetWeekNumber(new DateTime(date.Year - 1, DEC, LASTDAYOFDEC));
    }

    return result;
}

With this function available, it is a piece of cake to get the first and last date for a certain date’s week, as such:

/// <summary>
/// Get the first date of the week for a certain date, provided
/// that the first day of the week is Monday, the first week of
/// a year is the one that includes the first Thursday of that
/// year and the last week of a year is the one that immediately
/// precedes the first calendar week of the next year.
/// </summary>
/// <param name="date">ISO 8601 date of interest.</param>
/// <returns>The first week date.</returns>
public static DateTime GetFirstDateOfWeek(this DateTime date)
{
    if (date == DateTime.MinValue) {
        return date;
    }

    int week = date.GetWeekNumber();
    
    while (week == date.GetWeekNumber()) {
        date = date.AddDays(-1);
    }

    return date.AddDays(1);
}

and

/// <summary>
/// Get the last date of the week for a certain date, provided
/// that the first day of the week is Monday, the first week of
/// a year is the one that includes the first Thursday of that
/// year and the last week of a year is the one that immediately
/// precedes the first calendar week of the next year.
/// </summary>
/// <param name="date">ISO 8601 date of interest.</param>
/// <returns>The first week date.</returns>
public static DateTime GetLastDateOfWeek(this DateTime date)
{
    if (date == DateTime.MaxValue) {
        return date;
    }

    int week = date.GetWeekNumber();

    while (week == date.GetWeekNumber()) {
        date = date.AddDays(1);
    }

    return date.AddDays(-1);
}

I hope that this helps anyone having problem with retrieving the week number. If you want the latest version, checkout NExtra.