Converting UNIX Time

Surprisingly, when you read some values from the Windows Registry, they do not seem to be in a readable format:

$key = 'HKLM:\Software\Microsoft\Windows NT\CurrentVersion'
Get-ItemProperty $key | 
  Select-Object -ExpandProperty InstallDate

The InstallDate value returns as a large number. It turns out that Windows stores its install date in UNIX format which are the seconds since 1/1/1970. Here is a little function that converts this back to real dates:

function ConvertFrom-UnixTime {
  param(
      [Parameter(Mandatory=$true, ValueFromPipeline=$true)]
    [Int32]
    $UnixTime
  )
  begin {
    $startdate = Get-Date –Date '01/01/1970' 
  }
  process {
    $timespan = New-Timespan -Seconds $UnixTime
    $startdate + $timespan
  }
}

So now, you can correctly read the installation date from the registry:

$key = 'HKLM:\Software\Microsoft\Windows NT\CurrentVersion'
Get-ItemProperty $key | 
  Select-Object -ExpandProperty InstallDate |
  ConvertFrom-UnixTime

If you wanted to know how many days your Windows is old, add some more PowerShell cmdlets:

$key = 'HKLM:\Software\Microsoft\Windows NT\CurrentVersion'
Get-ItemProperty $key | 
  Select-Object -ExpandProperty InstallDate |
  ConvertFrom-UnixTime |
  New-TimeSpan |
  Select-Object -ExpandProperty Days

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