ISO8601 Weeknumber SOLVED!

[B-Art]

I have found a problem with getting the correct weeknumber via:

function Get-WeekNumber([datetime]$DT = (Get-Date)) {
	(Get-Culture).Calendar.GetWeekOfYear((Get-Date($DT)), ((Get-Culture).DateTimeFormat.CalendarWeekRule), ((Get-Culture).DateTimeFormat.FirstDayOfWeek))
}

Get-WeekNumber('2013-01-01')
Get-WeekNumber('2012-12-31')
Get-WeekNumber('2012-12-30')
Get-WeekNumber('2012-12-29')

OR

function Get-UIWeekNumber([datetime]$DT = (Get-Date)) {
	(Get-UICulture).Calendar.GetWeekOfYear((Get-Date($DT)), ((Get-UICulture).DateTimeFormat.CalendarWeekRule), ((Get-UICulture).DateTimeFormat.FirstDayOfWeek))
}

Get-UIWeekNumber('2013-01-01')
Get-UIWeekNumber('2012-12-31')
Get-UIWeekNumber('2012-12-30')
Get-UIWeekNumber('2012-12-29')

This does NOT give all the right answers.

I have tried to bugfix the problem and came up with solutions for Culture and UICulture:

     <#
     For Both Culture AND UICulture:
		First create an integer(0/1) from the boolean,
		"Is the integer DayOfWeek value greater than zero?".
		Then Multiply it with 4 or 6 (weekrule = 0 or 2) minus the integer DayOfWeek value.
		This turns every day (except Sunday) into Thursday.
		Then return the ISO8601 WeekNumber.
     #>

function Get-ISO8601Week {
     Param(
     [datetime]$DT = (Get-Date)
     )
     $Cult = Get-Culture; $DT = Get-Date($DT)
     $WeekRule = $Cult.DateTimeFormat.CalendarWeekRule
     $FirstDayOfWeek = $Cult.DateTimeFormat.FirstDayOfWeek
     $WeekRuleDay = [int]($DT.DayOfWeek.Value__ -ge $FirstDayOfWeek ) * ( (6 - $WeekRule) - $DT.DayOfWeek.Value__ )
     $Cult.Calendar.GetWeekOfYear(($DT).AddDays($WeekRuleDay), $WeekRule, $FirstDayOfWeek)
}

function Get-UiISO8601Week {
     Param(
     [datetime]$DT = (Get-Date)
     )
     $Cult = Get-UICulture; $DT = Get-Date($DT)
     $WeekRule = $Cult.DateTimeFormat.CalendarWeekRule
     $FirstDayOfWeek = $Cult.DateTimeFormat.FirstDayOfWeek
     $WeekRuleDay = [int]($DT.DayOfWeek.Value__ -ge $FirstDayOfWeek ) * ( (6 - $WeekRule) - $DT.DayOfWeek.Value__ )
     $Cult.Calendar.GetWeekOfYear(($DT).AddDays($WeekRuleDay), $WeekRule, $FirstDayOfWeek)
}

But it would better if the first function would work correct.

Both give the correct results when using:

Write-Host "UICulture: " -NoNewline
(20..31).ForEach( { Get-UiISO8601Week("2012-12-$($_)") }) + ((1..7).ForEach( { Get-UiISO8601Week("2013-1-$($_)") })) -join ', ' 
Write-Host "  Culture: " -NoNewline
(20..31).ForEach( { Get-ISO8601Week("2012-12-$($_)") }) + ((1..7).ForEach( { Get-ISO8601Week("2013-1-$($_)") })) -join ', '

Will give:

UICulture: 51, 51, 51, 52, 52, 52, 52, 52, 52, 52, 1, 1, 1, 1, 1, 1, 1, 2, 2
  Culture: 51, 51, 51, 51, 52, 52, 52, 52, 52, 52, 52, 1, 1, 1, 1, 1, 1, 1, 2

[huoyaoyuan]

Per ISO08061 standard, it uses FirstDayOfWeek = Monday and CalendarWeekRule = FirstFourDayWeek.
You should never touch CurrentCulture or CurrentUICulture because they may not follow ISO08061.

The correct script in Powershell should be:

[System.Globalization.CultureInfo]::InvariantCulture.Calendar.GetWeekOfYear((Get-Date($DT)), 'FirstFourDayWeek', 'Monday')

[B-Art]

This is not very different from my starting code.

[huoyaoyuan]

The key different is to specify 'FirstFourDayWeek' and 'Monday'. They are parts of ISO08061 standard. The default values specified by CurrentCulture are irrelevant.

[Clockwork-Muse]

I have found a problem with getting the correct weeknumber via:

... soooooo, what values are you getting, and what do you believe the correct values should be? "It's not working" is not a diagnosable, reproducible, problem (or at least, not reliably and easily). Especially in any area where culture settings are involved.

Note that I'm otherwise with @huoyaoyuan here - you probably need the invariant culture.

[huoyaoyuan]

With a little modification from my previous post:

$(New-Object System.Globalization.GregorianCalendar).GetWeekOfYear((Get-Date($DT)), 'FirstFourDayWeek', 'Monday')

[B-Art]

@huoyaoyuan Your code has a bug:

function Get-GregorianCalendarIso8601WeekNumber {
    Param(
        [datetime]$DT = (Get-Date)
    )
    $(New-Object System.Globalization.GregorianCalendar).GetWeekOfYear((Get-Date($DT)), 'FirstFourDayWeek', 'Monday')        
}

(20..31).ForEach( { Get-GregorianCalendarIso8601WeekNumber("2012-12-$($_)") }) + ((1..7).ForEach( { Get-GregorianCalendarIso8601WeekNumber("2013-1-$($_)") })) -join ', '

Gives:
51, 51, 51, 51, 52, 52, 52, 52, 52, 52, 52, 53, 1, 1, 1, 1, 1, 1, 2

[huoyaoyuan]

Well, ISO8601 formats this as 2013-W01-1. The GetWeekOfYear is unaware of this.

[B-Art]

@huoyaoyuan ISO8601 defenition is NOT the problem.
dotnet code has a bug.

[huoyaoyuan]

NO. Dotnet implementation doesn't say they are compatible with ISO8601. They are unaware. It your responsibility to change the year number.

[B-Art]

@Clockwork-Muse Still a bug...

And Culture or UICulture or InvariantCulture all the same bug.

[huoyaoyuan]

It's your bug, because the Calander isn't aware of ISO8601. You can't just get a compatible week number with 1 call. In this case you should adjust the week number too.

And you should neither touch CultureInfo nor add GregorianCalendar into the name. Using gregorian calander is a part of ISO8601 standard.

[Clockwork-Muse]

Okay, so, apparently you can't map exactly to ISO8601 via Calendar (down near the bottom).
You want the System.Globalization.ISOWeek class

[huoyaoyuan]

Thanks for pointing out this. I didn't search for that.

[Clockwork-Muse]

@huoyaoyuan - I mean, I didn't know this existed either, I just ran across it when looking at the calendar remarks. I think Java's date/time classes spoiled me.

[B-Art]

So I made a funtion:

function Get-WeekNumber {
    Param(
        [Parameter(
            # Mandatory,
            ValueFromPipeline,
            ValueFromPipelineByPropertyName,
            HelpMessage = 'Valid Date'
        )]
        [Alias('Date', 'DateTime')]
            [datetime[]]$Dates = (Get-Date)
    )

    BEGIN{
	$FirstDayOfWeek = (Get-Culture).DateTimeFormat.FirstDayOfWeek.value__
    }
    PROCESS{
        ForEach ($DateElement in $Dates) {
            Try {
                Write-Verbose "------------------------"
                Write-Verbose "Try to turn $DateElement into ISO8601Week"
                $WeekNumber = [System.Globalization.ISOWeek]::GetWeekOfYear($DateElement)
<#
Generate the week of day numbers 1..7 based on culture firstdayofweek value and then pick the day of today as an index
#>
		$DayNumber = (0..6).ForEach( { (($_ - $FirstDayOfWeek + 7) % 7) + 1 })[$DateElement.DayOfWeek.value__]
                $Properties = @{
			Date = Get-Date($DateElement)
			DayOfWeek = (Get-Date($DateElement)).DayOfWeek
			WeekRuleDay = $WeekRuleDay
			WeekNumber = $WeekNumber
			ISOWeek    = "$($DateElement.Year)-W$WeekNumber"
			ISOWeekDay = "$($DateElement.Year)-W$WeekNumber-$DayNumber"
			ISO8601Week    = "$($DateElement.Year)-W$WeekNumber"
			ISO8601WeekDay = "$($DateElement.Year)-W$WeekNumber-$DayNumber"
                }
            } Catch {
		Write-Verbose "Could not turn: $DateElement into ISO8601Week"
		$Properties = @{
			Date = $DateElement
			DayOfWeek = ''
			WeekRuleDay = ''
			WeekNumber = ''
			ISOWeek = ''
			ISOWeekDay = ''
			ISO8601Week = ''
			ISO8601WeekDay = ''
                }
            } Finally{
			Write-Verbose "Final"
			$Obj = New-Object -Typename PSCustomObject -Property $Properties
			Write-Output $Obj
            }
            
        }
    }
}

Following the ISO standard the First day of the week has Number 1 so on to Number 7

(20..31).ForEach( { (Get-WeekNumber("2012-12-$($_)")).ISOWeekDay }) + ((1..7).ForEach( { (Get-WeekNumber("2013-1-$($_)")).ISOWeekDay })) -join ','

Will output:

2012-W51-4,2012-W51-5,2012-W51-6,2012-W51-7,2012-W52-1,2012-W52-2,2012-W52-3,2012-W52-4,2012-W52-5,2012-W52-6,2012-W52-7,2012-W1-1,2013-W1-2,2013-W1-3,2013-W1-4,2013-W1-5,2013-W1-6,2013-W1-7,2013-W2-1

And today:

Get-WeekNumber

Gave:

ISOWeek        : 2021-W6
WeekNumber     : 6
ISO8601Week    : 2021-W6
Date           : 2021-02-10 19:04:02
ISO8601WeekDay : 2021-W6-4
ISOWeekDay     : 2021-W6-4