Add week number format tokens

DesktopClock.Tests/TimeStringFormatterTests.cs

@@ -133,6 +133,24 @@ public void Format_ConvertsUsingSelectedTimeZone()
         Assert.Equal("18:30", result);
     }
 
+    [Fact]
+    public void Format_WeekTokenUsesSelectedTimeZone()
+    {
+        var now = new DateTimeOffset(2024, 12, 29, 23, 30, 0, TimeSpan.Zero);
+        var timeZone = TimeZoneInfo.CreateCustomTimeZone("UtcPlus2", TimeSpan.FromHours(2), "UtcPlus2", "UtcPlus2");
+
+        var result = TimeStringFormatter.Format(
+            now,
+            now.DateTime,
+            timeZone,
+            default,
+            "{weekYear}-W{week}",
+            null,
+            CultureInfo.InvariantCulture);
+
+        Assert.Equal("2025-W1", result);
+    }
+
     [Fact]
     public void Format_InvalidClockFormatShowsBriefErrorMessage()
     {

DesktopClock.Tests/TokenizerTests.cs

@@ -111,6 +111,41 @@ public void FormatWithTokenizer_DateTimeOffset_ShouldWork()
         Assert.Equal("Sunday, Sep 24", result);
     }
 
+    [Theory]
+    [InlineData("2024-01-01", "Week 1")]
+    [InlineData("2024-12-29", "Week 52")]
+    [InlineData("2024-12-30", "Week 1")]
+    [InlineData("2021-01-01", "Week 53")]
+    public void FormatWithTokenizer_WeekToken_ShouldUseIsoWeek(string date, string expected)
+    {
+        // Arrange
+        var dateTime = DateTime.Parse(date, CultureInfo.InvariantCulture);
+        var format = "Week {week}";
+
+        // Act
+        var result = Tokenizer.FormatWithTokenizerOrFallBack(dateTime, format, CultureInfo.InvariantCulture);
+
+        // Assert
+        Assert.Equal(expected, result);
+    }
+
+    [Theory]
+    [InlineData("2024-12-30", "2025-W1")]
+    [InlineData("2021-01-01", "2020-W53")]
+    [InlineData("2026-05-06", "2026-W19")]
+    public void FormatWithTokenizer_WeekYearToken_ShouldUseIsoWeekYear(string date, string expected)
+    {
+        // Arrange
+        var dateTime = DateTime.Parse(date, CultureInfo.InvariantCulture);
+        var format = "{weekYear}-W{week}";
+
+        // Act
+        var result = Tokenizer.FormatWithTokenizerOrFallBack(dateTime, format, CultureInfo.InvariantCulture);
+
+        // Assert
+        Assert.Equal(expected, result);
+    }
+
     [Fact]
     public void FormatWithTokenizer_StandardFormat_WithoutBraces_ShouldWork()
     {

DesktopClock/Utilities/DateFormatExample.cs

@@ -58,6 +58,8 @@ public static DateFormatExample FromFormat(string format, DateTimeOffset dateTim
         "{dddd}, {MMM dd}, {h:mm tt}",        // Custom format: "Monday, Apr 10, 2:30 PM"
         "{dddd}, {MMM dd}, {HH:mm:ss}",       // Custom format: "Monday, Apr 10, 14:30:45"
         "{dddd}, {MMM dd}, {h:mm:ss tt}",     // Custom format: "Monday, Apr 10, 2:30:45 PM"
+        "Week {week}",                        // Custom token: "Week 15"
+        "Week {week}, {weekYear}",            // Custom token: "Week 15, 2023"
 
         // Standard formats
         "D",                                  // Long date pattern: Monday, June 15, 2009 (en-US)

DesktopClock/Utilities/Tokenizer.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Globalization;
 using System.Text.RegularExpressions;
 
 namespace DesktopClock;
@@ -31,7 +32,7 @@ public static string FormatWithTokenizerOrFallBack(IFormattable formattable, str
                     return _tokenizerRegex.Replace(format, (m) =>
                     {
                         var formatString = m.Groups[1].Value;
-                        return formattable.ToString(formatString, formatProvider);
+                        return FormatToken(formattable, formatString, formatProvider);
                     });
                 }
 
@@ -48,6 +49,75 @@ public static string FormatWithTokenizerOrFallBack(IFormattable formattable, str
         return formattable.ToString();
     }
 
+    private static string FormatToken(IFormattable formattable, string format, IFormatProvider formatProvider)
+    {
+        if (TryFormatWeekToken(formattable, format, out var result))
+        {
+            return result;
+        }
+
+        return formattable.ToString(format, formatProvider);
+    }
+
+    private static bool TryFormatWeekToken(IFormattable formattable, string format, out string result)
+    {
+        result = null;
+
+        if (!TryGetDateTime(formattable, out var dateTime))
+        {
+            return false;
+        }
+
+        switch (format)
+        {
+            case "week":
+                result = GetIsoWeek(dateTime).ToString(CultureInfo.InvariantCulture);
+                return true;
+
+            case "weekYear":
+                result = GetIsoWeekYear(dateTime).ToString(CultureInfo.InvariantCulture);
+                return true;
+
+            default:
+                return false;
+        }
+    }
+
+    private static bool TryGetDateTime(IFormattable formattable, out DateTime dateTime)
+    {
+        switch (formattable)
+        {
+            case DateTime value:
+                dateTime = value;
+                return true;
+
+            case DateTimeOffset value:
+                dateTime = value.DateTime;
+                return true;
+
+            default:
+                dateTime = default;
+                return false;
+        }
+    }
+
+    private static int GetIsoWeek(DateTime dateTime)
+    {
+        if (dateTime.DayOfWeek is >= DayOfWeek.Monday and <= DayOfWeek.Wednesday)
+        {
+            dateTime = dateTime.AddDays(3);
+        }
+
+        var calendar = CultureInfo.InvariantCulture.Calendar;
+        return calendar.GetWeekOfYear(dateTime, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
+    }
+
+    private static int GetIsoWeekYear(DateTime dateTime)
+    {
+        var day = dateTime.DayOfWeek == DayOfWeek.Sunday ? 7 : (int)dateTime.DayOfWeek;
+        return dateTime.AddDays(4 - day).Year;
+    }
+
     private static bool UsesTokenSyntax(string format)
     {
         return format.Contains("{") || format.Contains("}");