Powershell - Get date from week and weekday - powershell

Question

I have a file containing 'year', 'week-number', and 'weekday', (example - '2022', '42', '4',) 42 represents week 42 of 2022, and the 4 represents tuesday. A 7 would represent sunday.
The first week is to be considered the first days until sunday, regardless if it is 7 days or not. Same principle for the last week. That week can also contain fewer than 7 days.
How can i translate this to a date like 'yyyyMMdd'?
Thanks

Answer 1

Note:
The following uses .NET's System.DayOfWeek enumeration to identify weekdays, which means that Sunday is 0, Monday is 1, ..., and Saturday is 6.
If your weekday-numbering scheme differs (which is what it sounds like), you'll have to map it onto the above.
# Convert the strings (representing CSV input) to numbers.
[int] $year, [int] $week, [int] $weekday = '2022', '42', '4'
# The day of the week you consider the start of a calendar week.
$startOfWeekDay = [DayOfWeek]::Monday
# Calculate the start of calendar week 1 for the given year:
# The Monday on or preceding Jan 1
# Note: This date may therefore fall into the previous year.
$Jan1 = [datetime]::new($year, 1, 1)
$StartOfWeek1 = $Jan1.AddDays(-((7 + $Jan1.DayOfWeek-$startOfWeekDay) % 7))
# Calculate the start of the target calendar week.
$StartOfTargetWeek = $StartOfWeek1.AddDays($week * 7)
# Calculate the target date by determining the desired weekday
# inside the target week.
$TargetDate = $StartOfTargetWeek.AddDays((7 + $weekday-$startOfWeekDay) % 7)
$TargetDateString = $TargetDate.ToString('yyyyMMdd')
# Output *for display* both the intermediate results and the final one.
[pscustomobject] #{
Jan1 = ($Jan1 | Out-String).Trim()
StartOfWeek1 = ($StartOfWeek1 | Out-String).Trim()
StartOfTargetWeek = ($StartOfTargetWeek | Out-String).Trim()
TargetWeekDay = [DayOfWeek] $weekday
TargetDate = ($TargetDate | Out-String).Trim()
TargetDateString = $TargetDateString
} | Format-List
Output:
Jan1 : Saturday, January 1, 2022 12:00:00 AM
StartOfWeek1 : Monday, December 27, 2021 12:00:00 AM
StartOfTargetWeek : Monday, October 17, 2022 12:00:00 AM
TargetWeekDay : Thursday
TargetDate : Thursday, October 20, 2022 12:00:00 AM
TargetDateString : 20221020