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Kreta.BusinessLogic/Classes/DateHelper.cs

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+using System;
+using System.Globalization;
+
+namespace Kreta.BusinessLogic.Classes
+{
+    public static class DateHelper
+    {
+        // This presumes that weeks start with Monday.
+        // Week 1 is the 1st week of the year with a Thursday in it.
+        public static int GetISO8601WeekOfYear(this DateTime date)
+        {
+            var calendar = CultureInfo.InvariantCulture.Calendar;
+
+            // Seriously cheat.  If its Monday, Tuesday or Wednesday, then it’ll 
+            // be the same week# as whatever Thursday, Friday or Saturday are,
+            // and we always get those right
+            DayOfWeek day = calendar.GetDayOfWeek(date);
+            if (DayOfWeek.Monday <= day && day <= DayOfWeek.Wednesday)
+            {
+                date = date.AddDays(3);
+            }
+
+            // Return the week of our adjusted day
+            return calendar.GetWeekOfYear(date, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
+        }
+
+        public static int GetISO8601WeekOfYear(this DateTime? date)
+        {
+            if (date.HasValue)
+            {
+                return date.Value.GetISO8601WeekOfYear();
+            }
+
+            return GetWeeksInYear();
+        }
+
+        private static int GetWeeksInYear()
+        {
+            return GetISO8601WeekOfYear(new DateTime(DateTime.Now.Year, 12, 31));
+        }
+    }
+}