The third value passed to diffForHumans() is for shortening the display output.
Try something like,
$post->created_at->diffForHumans(null, false, true)
here you can see a the comments for diffForHumans() and the values it accepts.
/**
* Get the difference in a human readable format in the current locale.
*
* When comparing a value in the past to default now:
* 1 hour ago
* 5 months ago
*
* When comparing a value in the future to default now:
* 1 hour from now
* 5 months from now
*
* When comparing a value in the past to another value:
* 1 hour before
* 5 months before
*
* When comparing a value in the future to another value:
* 1 hour after
* 5 months after
*
* @param Carbon|null $other
* @param bool $absolute removes time difference modifiers ago, after, etc
* @param bool $short displays short format of time units
* @param int $parts displays number of parts in the interval
*
* @return string
*/
public function diffForHumans($other = null, $absolute = false, $short = false, $parts = 1)
{