cannot generate authenticator token every 15secs instead of default 30secs

I am not able to reduce the default time limit of 30secs to 15 secs to generate authenticator token, below is the code I am trying.

```
otplib.authenticator.options={
   epoch: Date.now(),
  step: 15,
  window: 0,
}
const code = otplib.authenticator.generate(secret);
```

please suggest
