From cbc8a1614c4e6512973c40fed570cc18eb995f2a Mon Sep 17 00:00:00 2001
From: Brian Lalonde <brianary@gmail.com>
Date: Mon, 1 Jan 2024 09:58:51 -0800
Subject: [PATCH] Get-Date -UFormat %W does not match reference implementation
 Fixes #20978

---
 .../commands/utility/GetDateCommand.cs        | 11 ++-
 .../Get-Date.Tests.ps1                        | 69 +++++++++++++++++++
 2 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/src/Microsoft.PowerShell.Commands.Utility/commands/utility/GetDateCommand.cs b/src/Microsoft.PowerShell.Commands.Utility/commands/utility/GetDateCommand.cs
index 62a2d276ebf6..caa547db1f7e 100644
--- a/src/Microsoft.PowerShell.Commands.Utility/commands/utility/GetDateCommand.cs
+++ b/src/Microsoft.PowerShell.Commands.Utility/commands/utility/GetDateCommand.cs
@@ -524,7 +524,16 @@ private string UFormatDateString(DateTime dateTime)
                             break;
 
                         case 'W':
-                            sb.Append(dateTime.DayOfYear / 7);
+                            var week = CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(
+                                dateTime, CalendarWeekRule.FirstFullWeek, DayOfWeek.Monday);
+                            if(dateTime.DayOfYear < 7 && week > 50)
+                            {
+                                sb.Append("00");
+                            }
+                            else
+                            {
+                                sb.AppendFormat("{0:00}", week);
+                            }
                             break;
 
                         case 'w':
diff --git a/test/powershell/Modules/Microsoft.PowerShell.Utility/Get-Date.Tests.ps1 b/test/powershell/Modules/Microsoft.PowerShell.Utility/Get-Date.Tests.ps1
index f72f1fc4ddb9..b7fc294ac225 100644
--- a/test/powershell/Modules/Microsoft.PowerShell.Utility/Get-Date.Tests.ps1
+++ b/test/powershell/Modules/Microsoft.PowerShell.Utility/Get-Date.Tests.ps1
@@ -55,6 +55,75 @@ Describe "Get-Date DRT Unit Tests" -Tags "CI" {
         Get-Date -Date 1/1/0030 -UFormat %S%T%u%U%w%W%x%X%y%Y%% | Should -Be "0000:00:00202001/01/3000:00:00300030%"
     }
 
+    # Alternate 'week of year' test cases, generated using `date -d $date +%W` on linux
+    It "using -uformat 'W' produces the correct output (<date>:<week>)" -TestCases @(
+        @{date="1999-01-03"; week = "00"},
+        @{date="1999-01-04"; week = "01"},
+        @{date="1999-02-14"; week = "06"},
+        @{date="1999-02-15"; week = "07"},
+        @{date="1999-12-21"; week = "51"},
+        @{date="1999-12-22"; week = "51"},
+        @{date="1999-12-27"; week = "52"},
+        @{date="1999-12-28"; week = "52"},
+        @{date="1999-12-29"; week = "52"},
+        @{date="2000-01-01"; week = "00"},
+        @{date="2000-01-02"; week = "00"},
+        @{date="2000-01-03"; week = "01"},
+        @{date="2000-01-04"; week = "01"},
+        @{date="2000-12-20"; week = "51"},
+        @{date="2000-12-21"; week = "51"},
+        @{date="2000-12-25"; week = "52"},
+        @{date="2000-12-26"; week = "52"},
+        @{date="2000-12-27"; week = "52"},
+        @{date="2000-12-28"; week = "52"},
+        @{date="2001-01-01"; week = "01"},
+        @{date="2001-01-02"; week = "01"},
+        @{date="2005-12-28"; week = "52"},
+        @{date="2005-12-29"; week = "52"},
+        @{date="2006-01-01"; week = "00"},
+        @{date="2006-01-02"; week = "01"},
+        @{date="2006-10-13"; week = "41"},
+        @{date="2006-10-16"; week = "42"},
+        @{date="2012-12-28"; week = "52"},
+        @{date="2012-12-31"; week = "53"},
+        @{date="2013-01-01"; week = "00"},
+        @{date="2013-01-02"; week = "00"},
+        @{date="2023-12-29"; week = "52"},
+        @{date="2024-01-01"; week = "01"},
+        @{date="2024-12-28"; week = "52"},
+        @{date="2024-12-30"; week = "53"},
+        @{date="2024-12-31"; week = "53"},
+        @{date="2025-01-01"; week = "00"},
+        @{date="2025-01-02"; week = "00"},
+        @{date="2025-01-03"; week = "00"},
+        @{date="2025-01-04"; week = "00"},
+        @{date="2025-01-05"; week = "00"},
+        @{date="2025-01-06"; week = "01"},
+        @{date="2025-03-09"; week = "09"},
+        @{date="2025-03-10"; week = "10"},
+        @{date="2025-03-17"; week = "11"},
+        @{date="2025-03-24"; week = "12"},
+        @{date="2025-03-31"; week = "13"},
+        @{date="2025-04-07"; week = "14"},
+        @{date="2025-04-14"; week = "15"},
+        @{date="2025-10-20"; week = "42"},
+        @{date="2025-10-27"; week = "43"},
+        @{date="2025-11-03"; week = "44"},
+        @{date="2025-11-10"; week = "45"},
+        @{date="2025-11-17"; week = "46"},
+        @{date="2025-11-24"; week = "47"},
+        @{date="2025-12-01"; week = "48"},
+        @{date="2025-12-08"; week = "49"},
+        @{date="2025-12-15"; week = "50"},
+        @{date="2025-12-22"; week = "51"},
+        @{date="2025-12-29"; week = "52"},
+        @{date="2026-01-01"; week = "00"},
+        @{date="2026-01-02"; week = "00"}
+    ) {
+        param($date, $week)
+        Get-Date -Date $date -UFormat %W | Should -BeExactly $week
+    }
+
     # The 'week of year' test cases is from https://en.wikipedia.org/wiki/ISO_week_date
     It "using -uformat 'V' produces the correct output (<date>:<week>)" -TestCases @(
         @{date="1998-01-02"; week = "01"},