This function gets the last boot up time of one or more computers from WMI.

Get-LastReboot.ps1

Function Get-LastReboot{
 <#
    .SYNOPSIS
    Get last reboot time.
    .DESCRIPTION
    This function gets the last boot up time of a computer from WMI. It can accept both pipeline input and parameter input.
    .PARAMETER $ComputerName
    Required. Name of the Computer to return data for. 
    .EXAMPLE
    Get-LastReboot -ComputerName localhost
    Retrieve last reboot time for a single computer
    .EXAMPLE
    Get-LastReboot -ComputerName localhost,dc
    Retrieve last reboot time for multiple computers
    .EXAMPLE
    Get-Content .\Servers.txt | Get-LastReboot
    Retrieve last reboot time for multiple computers, using pipeline input
    .NOTES
    Author: Lee Sartorelli 
    Created: 4/4/2017
    ForEach loop is required to support comma separated values as a parameter input.
 #> 
    [CmdletBinding()]
    Param(
        [Parameter(Mandatory,
                ValueFromPipeline)]
        [Alias('Display Name','Server','host''CN')]
        [string[]]$ComputerName
    )
 
    Begin {}
 
    Process 
        {
            ForEach($Computer in $ComputerName)
                {
                $LastBoot = (Get-WMIObject Win32_OperatingSystem -ComputerName $Computer).LastBootUpTime
                $LastBoot = [Management.ManagementDateTimeConverter]::ToDateTime($LastBoot)
                    Write-Output "$Computer was last rebooted $LastBoot"
                }
        }
        
 
    End {}

} #Function Get-LastReboot