Get-LastRebootTime.ps1

Function Get-LastRebootTime
{
    [CmdLetBinding()]
    param
    (
        [parameter(Mandatory)]
        [string] $ResourceGroupName,

        [string] $VMName
    )
    if (-not ($VMName))
    {
        $VMList = Get-AzVM -ResourceGroupName $ResourceGroupName
    }
    else 
    {
        $VMList = Get-AzVM -ResourceGroupName -Name $VMName    
    }

    $Output = @()

    $Output += foreach ($VM in $VMList)
    {
        $ComputerInfo =  Get-CimInstance -ClassName CIM_OperatingSystem -Property 'LastBootUpTime' -ComputerName $VM.Name
        New-Object -TypeName psobject -Property @{
            ComputerName    = $VM.Name
            $LastRebootTime = $ComputerInfo.LastBootUpTime
        }
    }

    return $Output
}