Getting Active Directory Last Backup Time using PowerShell

function Convert-TimeToDays {
    [CmdletBinding()]
    param (
        $StartTime,
        $EndTime,
        #[nullable[DateTime]] $StartTime, # can't use this just yet, some old code uses strings in StartTime/EndTime.
        #[nullable[DateTime]] $EndTime, # After that's fixed will change this.
        [string] $Ignore = '*1601*'
    )
    if ($null -ne $StartTime -and $null -ne $EndTime) {
        try {
            if ($StartTime -notlike $Ignore -and $EndTime -notlike $Ignore) {
                $Days = (NEW-TIMESPAN -Start $StartTime -End $EndTime).Days
            }
        } catch {}
    } elseif ($null -ne $EndTime) {
        if ($StartTime -notlike $Ignore -and $EndTime -notlike $Ignore) {
            $Days = (NEW-TIMESPAN -Start (Get-Date) -End ($EndTime)).Days
        }
    } elseif ($null -ne $StartTime) {
        if ($StartTime -notlike $Ignore -and $EndTime -notlike $Ignore) {
            $Days = (NEW-TIMESPAN -Start $StartTime -End (Get-Date)).Days
        }
    }
    return $Days
}
function Get-WinADLastBackup {
    <#
    .SYNOPSIS
    Gets Active directory forest or domain last backup time

    .DESCRIPTION
    Gets Active directory forest or domain last backup time

    .PARAMETER Domain
    Optionally you can pass Domains by hand

    .EXAMPLE
    $LastBackup = Get-WinADLastBackup
    $LastBackup | Format-Table -AutoSize

    .EXAMPLE
    $LastBackup = Get-WinADLastBackup -Domain 'ad.evotec.pl'
    $LastBackup | Format-Table -AutoSize

    .NOTES
    General notes
    #>

    [cmdletBinding()]
    param(
        [string[]] $Domains
    )
    $NameUsed = [System.Collections.Generic.List[string]]::new()
    [DateTime] $CurrentDate = Get-Date
    if (-not $Domains) {
        try {
            $Forest = Get-ADForest -ErrorAction Stop
            $Domains = $Forest.Domains
        } catch {
            Write-Warning "Get-WinADLastBackup - Failed to gather Forest Domains $($_.Exception.Message)"
        }
    }
    foreach ($Domain in $Domains) {
        try {
            [string[]]$Partitions = (Get-ADRootDSE -Server $Domain -ErrorAction Stop).namingContexts
            [System.DirectoryServices.ActiveDirectory.DirectoryContextType] $contextType = [System.DirectoryServices.ActiveDirectory.DirectoryContextType]::Domain
            [System.DirectoryServices.ActiveDirectory.DirectoryContext] $context = New-Object System.DirectoryServices.ActiveDirectory.DirectoryContext($contextType, $Domain)
            [System.DirectoryServices.ActiveDirectory.DomainController] $domainController = [System.DirectoryServices.ActiveDirectory.DomainController]::FindOne($context)
        } catch {
            Write-Warning "Get-WinADLastBackup - Failed to gather partitions information for $Domain with error $($_.Exception.Message)"
        }

        $Output = ForEach ($Name in $Partitions) {
            if ($NameUsed -contains $Name) {
                continue
            } else {
                $NameUsed.Add($Name)
            }
            $domainControllerMetadata = $domainController.GetReplicationMetadata($Name)
            $dsaSignature = $domainControllerMetadata.Item("dsaSignature")
            $LastBackup = [DateTime] $($dsaSignature.LastOriginatingChangeTime)
            [PSCustomObject] @{
                Domain            = $Domain
                NamingContext     = $Name
                LastBackup        = $LastBackup
                LastBackupDaysAgo = - (Convert-TimeToDays -StartTime ($CurrentDate) -EndTime ($LastBackup))
            }
        }
        $Output
    }
}

$LastBackup = Get-WinADLastBackup
$LastBackup | Format-Table -AutoSize