✨ Add ISO week conversion

ConvertFrom-IsoWeekDate.ps1

@@ -0,0 +1,21 @@
+<#
+.SYNOPSIS
+Returns a DateTime object from an ISO week date string.
+#>
+
+#Requires -Version 3
+[CmdletBinding()] Param(
+[Parameter(Position=0,ValueFromPipeline=$true)][ValidatePattern('\A\d+-W\d\d-\d\z')][string] $InputObject
+)
+Process
+{
+	if($InputObject -notmatch '\A(?<Year>\d+)-W(?<Week>\d\d)-(?<DayOfWeek>\d)\z')
+	{
+		Stop-ThrowError.ps1 "Unable to parse '$InputObject' as an ISO week date." -Argument InputObject
+	}
+	$year,$week,$dow = [int]$Matches.Year,[int]$Matches.Week,[int]$Matches.DayOfWeek
+	$value = New-Object DateTime $year,1,1
+	$startdow = [int]$value.DayOfWeek
+	if($startdow -gt 4) {return $value.AddDays(7*$week+$dow+(7-$startdow))}
+	else {return $value.AddDays(7*$week+$dow-$startdow)}
+}