The following can do this (there seems to be an error in your code).
It assumes that you have the Posh-SSH module installed (see Use Posh-SSH instead of PuTTY)
$esxName = 'MyEsx'$cred = Get-Credential -Message "Credentials for $esxName"
$esx = Get-VMHost -Name $esxName
$cmdSub = @'
days=$(awk -F":" '$1 == "root" {print $3}' /etc/shadow)
seconds_since_epoch=$((days*60*60*24))
date --date=@$((seconds_since_epoch))
'@
$session = New-SSHSession -ComputerName $esx.Name -Credential $cred -AcceptKey
$result = Invoke-SSHCommand -SSHSession $session -Command $cmdSub
Remove-SSHSession -SSHSession $session | Out-Null
$result.Output
