$MyDate = '2021-01-01'


# Author:QQ-5201351
$DateObj = [datetime]::ParseExact($MyDate, 'yyyy-MM-dd', [System.Globalization.CultureInfo]::InvariantCulture)

# 获取该日期是星期几（Monday=1, ..., Sunday=7）
$dayOfWeek = [int]$DateObj.DayOfWeek
if ($dayOfWeek -eq 0) { $dayOfWeek = 7 }  # 将 Sunday 从 0 改为 7

# 计算本周的星期四（Thursday 是 4）
$thursdayOffset = 4 - $dayOfWeek
$thursday = $DateObj.AddDays($thursdayOffset)

# ISO 年份 = 星期四所在的公历年份
$isoYear = $thursday.Year

# 使用 GregorianCalendar 计算周数（基于 Monday + FirstFourDayWeek）
$calendar = [System.Globalization.GregorianCalendar]::new()
$isoWeek = $calendar.GetWeekOfYear(
    $DateObj,
    [System.Globalization.CalendarWeekRule]::FirstFourDayWeek,
    [DayOfWeek]::Monday
)

# 格式化输出（周数补零到两位）
Write-Output ("{0}{1:D2}" -f $isoYear, $isoWeek)