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    <pre><code>var currentCulture = CultureInfo.CurrentCulture; var weekNo = currentCulture.Calendar.GetWeekOfYear( new DateTime(2013, 12, 31), currentCulture.DateTimeFormat.CalendarWeekRule, currentCulture.DateTimeFormat.FirstDayOfWeek); </code></pre> <p>Be aware that this is <strong>not</strong> <a href="http://en.wikipedia.org/wiki/ISO_8601#Week_dates" rel="noreferrer">ISO 8601</a> compatible. In Sweden we use ISO 8601 <a href="http://msdn.microsoft.com/en-us/library/system.globalization.calendar.getweekofyear.aspx" rel="noreferrer">week</a> numbers but even though the culture is set to "sv-SE", <code>CalendarWeekRule</code> is <code>FirstFourDayWeek</code>, and <code>FirstDayOfWeek</code> is Monday the <em>weekNo</em> variable will be set to <strong>53</strong> instead of the correct <strong>1</strong> in the above code.</p> <p>I have only tried this with Swedish settings but I'm pretty sure that all countries (Austria, Germany, Switzerland and more) using ISO 8601 week numbers will be affected by this problem.</p> <p><a href="http://codebetter.com/blogs/peter.van.ooijen/archive/2005/09/26/132466.aspx" rel="noreferrer">Peter van Ooijen</a> and <a href="http://blogs.msdn.com/shawnste/archive/2006/01/24/iso-8601-week-of-year-format-in-microsoft-net.aspx" rel="noreferrer">Shawn Steele</a> has different solutions to this problem.</p> <p>Here's a compact solution</p> <pre><code>private static int WeekOfYearISO8601(DateTime date) { var day = (int)CultureInfo.CurrentCulture.Calendar.GetDayOfWeek(date); return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date.AddDays(4 - (day == 0 ? 7 : day)), CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday); } </code></pre> <p>It's been tested for the following dates</p> <pre><code>var datesAndISO8601Weeks = new Dictionary&lt;DateTime, int&gt; { {new DateTime(2000, 12, 31), 52}, {new DateTime(2001, 1, 1), 1}, {new DateTime(2005, 1, 1), 53}, {new DateTime(2007, 12, 31), 1}, {new DateTime(2008, 12, 29), 1}, {new DateTime(2010, 1, 3), 53}, {new DateTime(2011, 12, 31), 52}, {new DateTime(2012, 1, 1), 52}, {new DateTime(2013, 1, 2), 1}, {new DateTime(2013, 12, 31), 1}, }; foreach (var dateWeek in datesAndISO8601Weeks) { Debug.Assert(WeekOfYearISO8601(dateWeek.Key) == dateWeek.Value, dateWeek.Key.ToShortDateString() + " should be week number " + dateWeek.Value + " but was " + WeekOfYearISO8601(dateWeek.Key)); } </code></pre>
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    1. POHow can I calculate/find the week-number of a given date?
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    1. USJonas Elfström
    1. USJonas Elfström
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    1. POHow can I calculate/find the week-number of a given date?
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    1. COVery nice answer. Thanks
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      1. USroosteronacid
    2. COsame problem with the rest of scandinavia I think, I'm danish and I was wondering why my tests showed 31/12 to be week #53 too as my Google Calender states its week #1.
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