protected function verifyChangedSince($value)
    {
        if (is_int($value)) {
            return $value;
        }
        // remove whitespaces, tabulation etc
        $trimmed = trim($value);
        if ($this->isValidTimeStamp($trimmed)) {
            return $trimmed;
        }
        //at this point we have numeric string which is not timestamp
        if (is_numeric($value)) {
            throw new SdkException("Input string doesn't look as unix timestamp or date string");
        }
        // trying to parse string into timestamp
        $converted = $this->convertToTimestamp($value);
        if (!$converted) {
            throw new SdkException("Input value should be unix timestamp or valid date string");
        }

        return $converted;
    }