Get the correct ISO8601 week number

Get-ISO8601Week

function Get-ISOWeek {
  Param(
		[Parameter(
			# Mandatory,
			ValueFromPipeline,
			ValueFromPipelineByPropertyName,
			HelpMessage = 'Valid Date'
		)]
		[Alias('DT', 'DateTime')]
			[datetime[]]$Dates = (Get-Date),
		[Validateset('Culture', 'UICulture', 'InvariantCulture')]
		[string]$Culture
	)
	<#
		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.
	#>
	BEGIN {
		$Cult = Switch ($Culture) {
			'UICulture' {
				Get-UICulture
				Break
			}
			'InvariantCulture' {
				[CultureInfo]::InvariantCulture
				Break
			}
			default{
				Get-Culture
			}
		}
		$WeekRule = $Cult.DateTimeFormat.CalendarWeekRule.value__
		$FirstDayOfWeek = $Cult.DateTimeFormat.FirstDayOfWeek.value__
	}
	PROCESS{
		ForEach ($DateElement in $Dates) {
			Try {
				Write-Verbose "------------------------"
				Write-Verbose "Try to turn $DateElement into ISO8601Week"
				$WeekRuleDay = [int]($DateElement.DayOfWeek.Value__ -ge $FirstDayOfWeek ) * ( (6 - $WeekRule) - $DateElement.DayOfWeek.Value__ )
				$WeekNumber = $Cult.Calendar.GetWeekOfYear(($DateElement).AddDays($WeekRuleDay), $WeekRule, $FirstDayOfWeek)
<#
So 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
			}
			
		}
	}
	END{}
}

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