PowerShell Get the Friday 12 Months From This Friday

I'm currently writing a PowerShell script for backup tapes and trying to calculate return dates of the tapes. Tapes are picked up and returned by our provider on Fridays. Retention periods are 4 weeks, 12 months and 3 years. Monthly tapesets fall on the first Monday of the month so I'm currently working with a monthly tapeset, which will return 12 months from now. How would I calculate 12 months from Friday 10/08/2021, but make sure the return date is also a Friday?

#Calculate Friday - Vaulting Date
$Friday = (Get-Date).AddDays(5-((Get-Date).DayOfWeek.value__)).ToString("MM/dd/yyyy")

#Calculate Return Dates
$Weekly = (Get-Date).AddDays(5-((Get-Date).DayOfWeek.value__)).AddDays(28).ToString("MM/dd/yyyy")
$Monthly = (Get-Date).AddDays(5-((Get-Date).DayOfWeek.value__)).AddMonths(12).ToString("MM/dd/yyyy")
$Yearly = (Get-Date).AddDays(5-((Get-Date).DayOfWeek.value__)).AddYears(3).ToString("MM/dd/yyyy")

$Monthly currently returns 10/08/2022 which is the Saturday after the date I want. I presume I'd run into the same problem with $Yearly as well, and what if it's a leap year? Any assistance would greatly be appreciated.

Solution

You were on the right track, except:

Your $someDate.AddDays(5-($someDate.DayOfWeek)) method of finding the Friday that falls into the same week as $someDate must be applied after the .AddMonths(), ... calls, i.e. to the results of the future-date-by-offset values.
- Note: Due to basing your calculation on the System.DayOfWeek enumeration, which ranges from value 0 (Sunday) to value 6 (Saturday), the implication is that, relative to the given date:
  - For Sunday through Thursday, you'll find the following Friday.
  - For Friday, you'll find that day itself.
  - For Saturday, you'll find the previous Friday, i.e. the day before.
Generally, as Mathias points out in his answer, you should work with [datetime] instances while performing date calculations and apply formatting (conversion to string representations) only to the calculation results.
- Note: [datetimeoffset] would even be better, due to unambiguously denoting absolute points in time;^[1] while you can work with this .NET type directly, PowerShell itself unfortunately doesn't offer first-class support for it (yet), such as via Get-Date; several feature requests are pending as of PowerShell 7.2 - see this GitHub search.

To put it all together:

# Get today's date without a time-of-day component (via .Date)
# Note: Not strictly necessary, if you use the results only as
#       formatted date-only strings.
$today = (Get-Date).Date

# Find this week's Friday.
$thisFriday = ($today).AddDays(5-($today.DayOfWeek))

# Add an offset.
$futureDate = $thisFriday.AddYears(1)

# Find the future date's adjacent Friday.
$futureFriday = $futureDate.AddDays(5-$futureDate.DayOfWeek)

# Output the result via an aux. object that results in tabular display.
[pscustomobject] @{
  ThisFriday = $thisFriday.ToString('MM/dd/yyyy')
  FutureFriday = $futureFriday.ToString('MM/dd/yyyy')
}

You'll see output such as the following:

ThisFriday FutureFriday
---------- ------------
10/08/2021 10/07/2022

^{[1] [datetimeoffset] is also DST-aware (aware of daylight-saving time) when performing calculations, whereas [datetime] appears not to be. This means that even when adding full days to [datetimeoffset] instances, a calendar-day boundary may be crossed if there is an intervening DST transition.}