PHP division resulting in NaN - maybe related to DateTime functions

I'm currently trying to do a "time ago" function but for some reason, it ends up giving me a NaN for one of the results.

I'm using a slightly modified version of this piece of code (found at: http://www.sitepoint.com/create-your-own-twitter-widget-3/ )

    if ($this->DateFormat == 'friendly') {
        // friendly date format
        $ival = $now->diff($stime);
        $yr = $ival->y;
        $mh = $ival->m + ($ival->d > 15);
        if ($mh > 11) $yr = 1;
        $dy = $ival->d + ($ival->h > 15);
        $hr = $ival->h;
        $mn = $ival->i + ($ival->s > 29);
        if ($yr > 0) {
            if ($yr == 1) $date = 'last year';
            else $date = $yr . ' years ago';
        }
        else if ($mh > 0) {
            if ($mh == 1) $date = 'last month';
            else $date = $mh . ' months ago';
        }
        else if ($dy > 0) {
            if ($dy == 1) $date = 'yesterday';
            else if ($dy < 8) $date = $dy . ' days ago';
            else if ($dy < 15) $date = 'last week';
            else $date = round($dy / 7) . ' weeks ago';
        }
        else if ($hr > 0) {
            $hr += ($ival->i > 29);
            $date = $hr . ' hour' . ($hr == 1 ? '' : 's') . ' ago';
        }
        else {
            if ($mn < 3) $date = 'just now';
            else $date = $mn . ' minutes ago';
        }
    }
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(the problematic line is the one with theround($dy / 7)

I created a set of possible values to test the outcomes and it gives me a NaN for a date of -29 days. The thing is that it only happens to do that when I repeat the function over and over. If I just test -29 days alone, it outputs me the correct 4 weeks value.

Any idea on how can I avoid/fix this?

Edit

This is most likely a bug in PHP.

If I make the code do the division and rounding twice, the NaN never appears.

// do estimate of how many weeks ago
$round = round($days / 7, 0);
$round = round($days / 7, 0);

$date =  $round . ' weeks ago';
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I noticed this because in my test loops, the first value that triggered the weeks calculation (given that I was making more than one calculation) would give the NaN.