Get-Date -UFormat %W does not match reference implementation

[brianary]

Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest released version
• Search the existing issues.
• Refer to the FAQ.
• Refer to Differences between Windows PowerShell 5.1 and PowerShell.

Steps to reproduce

Get-Date -UFormat %W

Currently, dateTime.DayOfYear / 7 is being used, but the code below is a better match. See Date conversion specifiers or strftime.

var startOfYearDayOfWeek = (int)(new DateTime(dateTime.Year, 1, 1)).DayOfWeek;
var adjustedDayOfYear = startOfYearDayOfWeek < 2 ? dateTime.DayOfYear + 7 : dateTime.DayOfYear;
var adjustedDayOfWeek = ((int)dateTime.DayOfWeek + 6) % 7;
sb.Append(((startOfYearDayOfWeek+adjustedDayOfYear-adjustedDayOfWeek)/7).ToString("00"));

Expected behavior

Based on running the `date -d $date +%W` as a reference implementation (see Environment for details).

PS> Get-Date 1999-01-01 -UFormat %W
00
PS> Get-Date 2000-02-14 -UFormat %W
07
PS> Get-Date 2001-03-29 -UFormat %W
13
PS> Get-Date 2002-05-12 -UFormat %W
18
PS> Get-Date 2003-06-25 -UFormat %W
25
PS> Get-Date 2004-08-07 -UFormat %W
31
PS> Get-Date 2005-09-20 -UFormat %W
38
PS> Get-Date 2006-11-03 -UFormat %W
44
PS> Get-Date 2007-12-17 -UFormat %W
51
PS> Get-Date 2009-01-29 -UFormat %W
04
PS> Get-Date 2010-03-14 -UFormat %W
10
PS> Get-Date 2011-04-27 -UFormat %W
17
PS> Get-Date 2012-06-09 -UFormat %W
23
PS> Get-Date 2013-07-23 -UFormat %W
29
PS> Get-Date 2014-09-05 -UFormat %W
35
PS> Get-Date 2015-10-19 -UFormat %W
42
PS> Get-Date 2016-12-01 -UFormat %W
48
PS> Get-Date 2018-01-14 -UFormat %W
02
PS> Get-Date 2019-02-27 -UFormat %W
08
PS> Get-Date 2020-04-11 -UFormat %W
14

Actual behavior

PS> Get-Date 1999-01-01 -UFormat %W
0
PS> Get-Date 2000-02-14 -UFormat %W
6
PS> Get-Date 2001-03-29 -UFormat %W
12
PS> Get-Date 2002-05-12 -UFormat %W
18
PS> Get-Date 2003-06-25 -UFormat %W
25
PS> Get-Date 2004-08-07 -UFormat %W
31
PS> Get-Date 2005-09-20 -UFormat %W
37
PS> Get-Date 2006-11-03 -UFormat %W
43
PS> Get-Date 2007-12-17 -UFormat %W
50
PS> Get-Date 2009-01-29 -UFormat %W
4
PS> Get-Date 2010-03-14 -UFormat %W
10
PS> Get-Date 2011-04-27 -UFormat %W
16
PS> Get-Date 2012-06-09 -UFormat %W
23
PS> Get-Date 2013-07-23 -UFormat %W
29
PS> Get-Date 2014-09-05 -UFormat %W
35
PS> Get-Date 2015-10-19 -UFormat %W
41
PS> Get-Date 2016-12-01 -UFormat %W
48
PS> Get-Date 2018-01-14 -UFormat %W
2
PS> Get-Date 2019-02-27 -UFormat %W
8
PS> Get-Date 2020-04-11 -UFormat %W
14

Error details

No response

Environment data

Name                           Value
----                           -----
PSVersion                      7.4.0
PSEdition                      Core
GitCommitId                    7.4.0
OS                             Microsoft Windows 10.0.22621
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

WSL Ubuntu details:

$ uname -a
Linux localhost 5.15.133.1-microsoft-standard-WSL2 #1 SMP Thu Oct 5 21:02:42 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

Visuals

No response