Try this:
$date = [datetime]'1/1/2013'
$computername = "Compter01"
$HotFix = Get-WmiObject win32_ntlogevent -filter "(logfile='system') AND (sourcename='ntservicepack')" -cn $computername |
Where {$_.ConvertToDateTime($_.TimeWritten) -gt $date} |
Select @{name="ServerName"; e={$_.__Server}},Message,@{label="Date Installed"; expression={$_.ConverttoDateTime($_.TimeWritten)}}
There may be a way to filter out dates using the WMI filter query but that's not my area of expertise.