This is a script that I use alot. It uses WMI to query a remote computer and return an object showing how long the computer have been running since last reboot.

#Requires -Version 2

[CmdletBinding()]

param(
[Parameter(ValueFromPipeline = $true, ValueFromPipelinebyPropertyName = $true)]
[String[]]$ComputerName = ([System.Net.Dns]::GetHostName()),
[string]$logfile = 'uptime-failed.txt'
)

BEGIN{Remove-Item $logfile -ErrorAction SilentlyContinue}

PROCESS {
foreach ($computer in $ComputerName){
$continue = $true
try {$wmi = Get-WmiObject -ComputerName $computer -ErrorAction Stop -Query "SELECT LastBootupTime FROM Win32_OperatingSystem"}
catch {
$continue = $false
[string]$wmiException = $_.Exception.Message
Write-Verbose $computer" - Query failed"
if ($wmiException){Write-Debug $computer" - $wmiException"}
$computer | Out-File $logfile
}
if ($continue){
$lastBootup = $wmi.ConvertToDateTime($wmi.LastBootupTime)
[TimeSpan]$uptime=New-TimeSpan $lastBootup $(Get-Date)
$obj = New-Object -TypeName PSObject
$obj | Add-Member -MemberType NoteProperty -Name ComputerName -Value $computer -PassThru |
Add-Member -MemberType NoteProperty -Name Days -Value $($uptime.days) -PassThru |
Add-Member -MemberType NoteProperty -Name Hours -Value $($uptime.hours) -PassThru |
Add-Member -MemberType NoteProperty -Name Minutes -Value $($uptime.minutes) -PassThru |
Add-Member -MemberType NoteProperty -Name Seconds -Value $($uptime.seconds)
Write-Output $obj}}}