public String previous() {
        return previous(System.currentTimeMillis() / 1000);
    }
Returns the TOTP code which would have been generated immediately prior
to the code currently being returned by generate().

@return
    The TOTP code which would have been generated immediately prior to
    the code currently being returned by generate().
extensions/guacamole-auth-totp/src/main/java/org/apache/guacamole/totp/TOTPGenerator.java