The following PowerShell gives you the date of the last successful Windows Update installation (works for me, but knowing when my system last installed updates, the time appears to be in UTC):

$updateSearch = (New-Object -ComObject Microsoft.Update.Session ).CreateUpdateSearcher()
($updateSearch.QueryHistory(0,$updateSearch.GetTotalHistoryCount())| ?{$_.ResultCode -eq 2}|Sort-Object -Property Date -Descending).Date|Select -First 1

For some more background into, quoting "https://stackoverflow.com/questions/33732541/powershell-how-to-get-date-of-last-windows-update-install-or-at-least-checked-f":

$Session = New-Object -ComObject Microsoft.Update.Session
$Searcher = $Session.CreateUpdateSearcher()
$HistoryCount = $Searcher.GetTotalHistoryCount()
# http://msdn.microsoft.com/en-us/library/windows/desktop/aa386532%28v=vs.85%29.aspx
$Searcher.QueryHistory(0,$HistoryCount) | ForEach-Object {$_}

This will list all updates which the system tried to install, including a timestamp (which on my system appears to be UTC) and a result code.
Those translate to:

0 = Not Started
1 = In Progress
2 = Succeeded
3 = Succeeded With Errrors
4 = Failed
5 = Aborted

If a patch failed to install several times, it is supposedly listed several times as well. I cannot confirm that, as I have no failed patches in my list.
It also appears that the large cumulative updates like the Creators Update resets the contents of the list. Mine only goes back to said Creators Update.
Either way, with a bit of scripting you can return a few more things, like the number of unique patches which have failed to install.

I am not sure if the status 0 could be used to determine how many patches the system knows about that are not installed yet.

If are you using windows 10:

Windows Update logs are now generated using ETW (Event Tracing for Windows).
Please run the Get-WindowsUpdateLog PowerShell command to convert ETW traces into a readable WindowsUpdate.log.
For more information, please visit http://go.microsoft.com/fwlink/?LinkId=518345

I had to figure out the hard way

Run this in your transformation properties and it will return you the date and time from the unix time provided by the poll.

strftime("%Y-%m-%d %H:%M", $1)

Cheers

it's time of last system update detection, download, and installation as UNIX time (seconds since epoch).

Best regards,
Victor