You do have to parse the events. This should get you started on what you're looking for:

$vmlist = Get-VM | Where {$_.PowerState -eq 'PoweredOff'}
ForEach ($vm in $vmlist){
  $vm | Get-VIEvent -Types Info | Where {$_.fullFormattedMessage -match "Powered Off"} | `
  Sort-Object -property createdTime |  select -last 1 | %{
  Write-Host $_.vm.name $_.createdTime | Out-Default}
}

The above assumes you've already done connect-viserver etc.