The downside here is that you're always performing this calculation since object literals always execute all keys. In the future and as a general good practice, opt for the least expensive approach:

```js
opts = opts || {};
opts.minUnit = opts.minUnit || 's';
opts.referenceDate = opts.referenceDate || Date.now();
```
Obviously `Date.now()` isn't a hugely expensive operation, but getting in the habit of using lazy operations will save your ass someday when you write that code and it calls a really heavy method..