Instead of doing a ForEach, use Select instead, and toss an impromptu hash table in to get your LastLogonTime converted like this:
Get-ADComputer -Filter * -SearchBase "OU=Computers,DC=domain,DC=local" -Properties Name,lastLogonTimestamp | select Name,@{l="Last Logon";e={[DateTime]::FromFileTime($_.lastLogonTimestamp)}}|Export-Csv time.csv -NoTypeInformation