public String previous(long time) {
        return generate(Math.max(startTime, time - timeStep));
    }
Returns the TOTP code which would have been generated immediately prior
to the code returned by invoking generate() with the given timestamp.

@param time
    The absolute timestamp to use to generate the TOTP code, in seconds
    since midnight, 1970-01-01, UTC (UNIX epoch).

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