$objFixes = Get-WmiObject Win32_QuickFixEngineering $objFixes | Select-Object CSName,HotFixID,@{Name="InstalledOn"; Expression={([datetime]$_.psbase.properties["InstalledOn"].Value).ToLocalTime()}}
The .ToLocalTime() is not required and was added to take the local timezone into account.
$objFixes = Get-WmiObject Win32_QuickFixEngineering $objFixes | Select-Object CSName,HotFixID,InstalledOn