greater than

I have the following code that works.  It will return the number of days since the last reboot.  $machine is a list of machines read from an AD OU.

#  this is done within a LOOP

$lastboot = Get-WMIObject -class win32_operatingsystem -ComputerName $machine

$lastboot.ConvertToDateTime($LBoot.LastBootUpTime)

I have tried several different things to highlight my output to Excel.

if ($lastboot -gt "*9*") {

 $c.Cells.Item($row, 9).Interior.ColorIndex 

 $c.Cells.Item($row, 9).Font.ColorIndex = 1 }

$c.Cells.Item($row, 9) = $lastboot

I've tried if ($lastboot -gt "*9*"), if ($lastboot -gt "9"), if ($lastboot -gt 9)  and nothing I try will change the Excel cell color.

Now, this works fine:

$lastboot = '100'

if ($lastboot -gt "*9*"){write-host "greater than 9"}

How come it does not work within my LOOP?