All registry keys have a value associated with called the Last Write Time. This is analogous to the last modification time for a file. When ever the registry key or one if its values has been created, modified, or deleted the value is updated to the current local system time. Unfortunately, there is no Last Write Time associated with a registry value, but it can be infered from the Last Write Time of the key.

Here is a PowerShell script to read the Last Write Time for a registry key.

Usage:

Get-RegKeyLastWriteTime.ps1 <Key> <SubKey>

Example:

Get-RegKeyLastWriteTime.ps1 HKLM SOFTWARE\Microsoft\Windows\CurrentVersion

Output:

Key                         LastWriteTime
—-                         ——————-
AdminDebug                  10/28/2009 7:50:51 PM
App Management              7/14/2009 4:41:12 AM
App Paths                   1/22/2010 2:07:18 PM
Applets                     7/14/2009 4:41:12 AM
Audio                       7/14/2009 4:41:12 AM
Authentication              7/14/2009 4:41:12 AM
BitLocker                   7/14/2009 4:41:12 AM
...

Get-RegKeyLastWriteTime.ps1 Script:

param (	[string] $Key, [string] $SubKey )

switch ($Key) {
    "HKCR" { $searchKey = 0x80000000} #HK Classes Root
    "HKCU" { $searchKey = 0x80000001} #HK Current User
    "HKLM" { $searchKey = 0x80000002} #HK Local Machine
    "HKU"  { $searchKey = 0x80000003} #HK Users
    "HKCC" { $searchKey = 0x80000005} #HK Current Config
    default { 
        #throw "Invalid Key. Use one of the following options HKCR, HKCU, HKLM, HKU, HKCC"
    }
}


$KEYQUERYVALUE = 0x1
$KEYREAD = 0x19
$KEYALLACCESS = 0x3F

$sig1 = @'
[DllImport("advapi32.dll", CharSet = CharSet.Auto)]
  public static extern int RegOpenKeyEx(
    int hKey,
    string subKey,
    int ulOptions,
    int samDesired,
    out int hkResult);
'@
$type1 = Add-Type -MemberDefinition $sig1 -Name Win32Utils `
    -Namespace RegOpenKeyEx -Using System.Text -PassThru

$sig2 = @'
[DllImport("advapi32.dll", EntryPoint = "RegEnumKeyEx")]
extern public static int RegEnumKeyEx(
    int hkey,
    int index,
    StringBuilder lpName,
    ref int lpcbName,
    int reserved,
    int lpClass,
    int lpcbClass,
    out long lpftLastWriteTime);
'@
$type2 = Add-Type -MemberDefinition $sig2 -Name Win32Utils `
    -Namespace RegEnumKeyEx -Using System.Text -PassThru

$sig3 = @'
[DllImport("advapi32.dll", SetLastError=true)]
public static extern int RegCloseKey(
    int hKey);
'@
$type3 = Add-Type -MemberDefinition $sig3 -Name Win32Utils `
    -Namespace RegCloseKey -Using System.Text -PassThru


$hKey = new-object int
$result = $type1::RegOpenKeyEx($searchKey, $SubKey, 0, $KEYREAD, [ref] $hKey)

#initialize variables
$builder = New-Object System.Text.StringBuilder 1024
$index = 0
$length = [int] 1024
$time = New-Object Long

#234 means more info, 0 means success. Either way, keep reading
while ( 0,234 -contains $type2::RegEnumKeyEx($hKey, $index++, `
    $builder, [ref] $length, $null, $null, $null, [ref] $time) )
{
    #create output object
    $o = "" | Select Key, LastWriteTime
    $o.Key = $builder.ToString()
    $o.LastWriteTime = (Get-Date $time).AddYears(1600)
    $o

    #reinitialize for next time through the loop  
    $length = [int] 1024
    $builder = New-Object System.Text.StringBuilder 1024
}

$result = $type3::RegCloseKey($hKey);