$retentionDays   = 4                       ## 4日分を保持
$backupRootPath  = "I:\Share01_Backup"
$fromPath        = "E:\Share01\"
$now             = Get-Date
$DateFolder      = $now.ToString("yyyyMMdd")
$limitDate       = ($now.AddDays(-$retentionDays)).ToString("yyyyMMdd")    ## 削除開始日付を得る
$toPath          = ("{0}\{1}"          -f $backupRootPath, $DateFolder)
$logPath         = ("{0}_robocopy.log" -f $toPath)

# 日付8桁フォルダを作る
New-Item -path $backupRootPath -Name $DateFolder -ItemType Directory -Force

# 日付フォルダにコピーを取る
ROBOCOPY $fromPath $toPath * /MIR /NP /NDL /FP /BYTES /LOG:$logPath

# 削除開始日付かそれより古い日付のフォルダ・ログを削除する
Get-ChildItem -Path $backupRootPath -Directory | 
    Where-Object {$_.Name -match '^\d{8}$' -and $_.Name -le $limitDate} |
        Foreach-Object {
          Remove-Item -Path  $_.FullName                        -Recurse -Force
          Remove-Item -Path ("{0}_robocopy.log" -f $_.FullName) -Recurse -ErrorAction Ignore
        }