Level 67
So you have a lastmod date like 2011-06-27T19:34:00+01:00.
$lastmod = Carbon\Carbon::parse('2011-06-27T19:34:00+01:00');
Then you can use any carbon commands to compare against now() (which is Laravels equivalent/shortcut to Carbon\Carbon::now()). You didn't say what you wanted to compare about it, but check the carbon docs. There are many ways (less than, greater than, elapsed days, elapsed seconds, etc.)
if ($lastmod->lt(now()) {} // is $lastmod less than right now
if (now()->diffInHours($lastmod) > 10) {} // is $lastmod older than 10 hours
etc.