Output data does not change

I have this code:

function Get-Uptime  {

$o = Get-WmiObject -Class win32_operatingsystem

(Get-Date).date - $o.ConvertToDateTime($o.LastBootUpTime)

}

get-uptime 

I have run this on 2 different Windows 7 machines.  And the same thing happens each time.  The output looks like this:

Days              : 1

Hours             : 9

Minutes           : 28

Seconds           : 3

Milliseconds      : 874

Ticks             : 1204838744010

TotalDays         : 1.39448928704861

TotalHours        : 33.4677428891667

TotalMinutes      : 2008.06457335

TotalSeconds      : 120483.874401

TotalMilliseconds : 120483874.401

But the output Never changes.  I run this command 10 times in a row and nothing changes.  How come?    I have even closed PowerShell ISE opened it back up and the same is the same.