From aada0a80b761e31875db38f5eb69fd1d34403402 Mon Sep 17 00:00:00 2001
From: Paal Braathen <paal.braathen@usit.uio.no>
Date: Tue, 14 Jan 2020 19:26:24 +0100
Subject: [PATCH] Use ISOWeek for week numbers in Get-Date (#11536)

---
 .../commands/utility/GetDateCommand.cs        | 29 +------------------
 .../Get-Date.Tests.ps1                        | 17 ++++++++++-
 2 files changed, 17 insertions(+), 29 deletions(-)

diff --git a/src/Microsoft.PowerShell.Commands.Utility/commands/utility/GetDateCommand.cs b/src/Microsoft.PowerShell.Commands.Utility/commands/utility/GetDateCommand.cs
index 066497950a..0b04096c93 100644
--- a/src/Microsoft.PowerShell.Commands.Utility/commands/utility/GetDateCommand.cs
+++ b/src/Microsoft.PowerShell.Commands.Utility/commands/utility/GetDateCommand.cs
@@ -474,34 +474,7 @@ namespace Microsoft.PowerShell.Commands
                             break;
 
                         case 'V':
-                            // .Net Core doesn't implement ISO 8601.
-                            // So we use workaround from https://blogs.msdn.microsoft.com/shawnste/2006/01/24/iso-8601-week-of-year-format-in-microsoft-net/
-                            // with corrections from comments
-
-                            // Culture doesn't matter since we specify start day of week
-                            var calender = CultureInfo.InvariantCulture.Calendar;
-                            var day = calender.GetDayOfWeek(dateTime);
-                            var normalizedDatetime = dateTime;
-
-                            switch (day)
-                            {
-                                case DayOfWeek.Monday:
-                                case DayOfWeek.Tuesday:
-                                case DayOfWeek.Wednesday:
-                                    normalizedDatetime = dateTime.AddDays(3);
-                                    break;
-
-                                case DayOfWeek.Friday:
-                                case DayOfWeek.Saturday:
-                                case DayOfWeek.Sunday:
-                                    normalizedDatetime = dateTime.AddDays(-3);
-                                    break;
-                            }
-
-                            // FirstFourDayWeek and DayOfWeek.Monday is from ISO 8601
-                            sb.Append(StringUtil.Format("{0:00}", calender.GetWeekOfYear(normalizedDatetime,
-                                                                                        CalendarWeekRule.FirstFourDayWeek,
-                                                                                        DayOfWeek.Monday)));
+                            sb.Append(StringUtil.Format("{0:00}", ISOWeek.GetWeekOfYear(dateTime)));
                             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 48a52d7940..6980d18028 100644
--- a/test/powershell/Modules/Microsoft.PowerShell.Utility/Get-Date.Tests.ps1
+++ b/test/powershell/Modules/Microsoft.PowerShell.Utility/Get-Date.Tests.ps1
@@ -52,6 +52,11 @@ Describe "Get-Date DRT Unit Tests" -Tags "CI" {
 
     # The 'week of year' test cases is from https://en.wikipedia.org/wiki/ISO_week_date
     It "using -uformat 'V' produces the correct output" -TestCases @(
+        @{date="1998-01-02"; week = "01"},
+        @{date="1998-01-03"; week = "01"},
+        @{date="2003-01-03"; week = "01"},
+        @{date="2004-01-02"; week = "01"},
+        @{date="2004-01-03"; week = "01"},
         @{date="2005-01-01"; week = "53"},
         @{date="2005-01-02"; week = "53"},
         @{date="2005-12-31"; week = "52"},
@@ -67,11 +72,21 @@ Describe "Get-Date DRT Unit Tests" -Tags "CI" {
         @{date="2008-12-30"; week = "01"},
         @{date="2008-12-31"; week = "01"},
         @{date="2009-01-01"; week = "01"},
+        @{date="2009-01-02"; week = "01"},
+        @{date="2009-01-03"; week = "01"},
         @{date="2009-12-31"; week = "53"},
         @{date="2010-01-01"; week = "53"},
         @{date="2010-01-02"; week = "53"},
         @{date="2010-01-03"; week = "53"},
-        @{date="2010-01-04"; week = "01"}
+        @{date="2010-01-04"; week = "01"},
+        @{date="2014-01-03"; week = "01"},
+        @{date="2015-01-02"; week = "01"},
+        @{date="2015-01-03"; week = "01"},
+        @{date="2020-01-03"; week = "01"},
+        @{date="2025-01-03"; week = "01"},
+        @{date="2026-01-02"; week = "01"},
+        @{date="2026-01-03"; week = "01"},
+        @{date="2031-01-03"; week = "01"}
     ) {
         param($date, $week)
         Get-date -Date $date -uformat %V | Should -BeExactly $week