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.
