npx quartz build --serve produced many warning messages regarding invalid date "0" in the .md files

running 
```bash
npx quartz build --serve
``` 

produces many warning messages like:

```
Warning: found invalid date "0" in `content/Playing the Game of Universal Adversarial Perturbations.md`. Supported formats: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date#date_time_string_format
```