If (!(Test-Path "HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Advanced")) {  
    New-Item -Path "HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Advanced" | Out-Null }  
Set-ItemProperty -Path "HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Advanced" -Name "ShowSecondsInSystemClock" -Type DWord -Value 1