public function humanDiff(int $from, int $to = null, string $format = ''): string
    {
        $to   = $to ?? time();
        $diff = (int) abs($to - $from);

        if ($format !== 'abbrev' && $format !== 'default') {
            $format = 'default'; // Must be valid.
        }
        if ($diff < $this::SECONDS_IN_MINUTE) {
            $diff            = max(1, $diff);
            $fmat['abbrev']  = ['%1$ss', '%1$ss'];
            $fmat['default'] = ['%1$s sec', '%1$s secs'];
        } elseif ($diff < $this::SECONDS_IN_HOUR) {
            $diff            = max(1, round($diff / $this::SECONDS_IN_MINUTE));
            $fmat['abbrev']  = ['%1$sm', '%1$sm'];
            $fmat['default'] = ['%1$s min', '%1$s mins'];
        } elseif ($diff < $this::SECONDS_IN_DAY) {
            $diff            = max(1, round($diff / $this::SECONDS_IN_HOUR));
            $fmat['abbrev']  = ['%1$sh', '%1$sh'];
            $fmat['default'] = ['%1$s hour', '%1$s hours'];
        } elseif ($diff < $this::SECONDS_IN_WEEK) {
            $diff            = max(1, round($diff / $this::SECONDS_IN_DAY));
            $fmat['abbrev']  = ['%1$sd', '%1$sd'];
            $fmat['default'] = ['%1$s day', '%1$s days'];
        } elseif ($diff < $this::SECONDS_IN_WEEK * 4) {
            $diff            = max(1, round($diff / $this::SECONDS_IN_WEEK));
            $fmat['abbrev']  = ['%1$sw', '%1$sw'];
            $fmat['default'] = ['%1$s week', '%1$s weeks'];
        } elseif ($diff < $this::SECONDS_IN_YEAR) {
            $diff            = max(1, round($diff / ($this::SECONDS_IN_WEEK * 4)));
            $fmat['abbrev']  = ['%1$smo', '%1$smo'];
            $fmat['default'] = ['%1$s month', '%1$s months'];
        } else {
            $diff            = max(1, round($diff / $this::SECONDS_IN_YEAR));
            $fmat['abbrev']  = ['%1$sy', '%1$sy'];
            $fmat['default'] = ['%1$s year', '%1$s years'];
        }
        return sprintf(_n($fmat[$format][0], $fmat[$format][1], $diff), $diff);
    }