One way is to cast it to [system.io.directoryinfo]
$a = Get-Date "02/09/2013 4:59 PM"
$d = [system.io.directoryinfo]"C:\Users\user\Documents\CV"
$d.LastWriteTime = $a
or using get-item
cmdlet
$a = Get-Date "02/09/2013 4:59 PM"
$d = get-item C:\Users\user\Documents\CV
$d.LastWriteTime = $a