date: `--date` always results in invalid date

As the title says, everything I tried to pass to the `--date` option results in invalid date.
```shell
$ date --version
date 0.0.7
$ date --date="Sun, 29 Feb 2004 16:21:42 -0800"
date: invalid date 'Sun, 29 Feb 2004 16:21:42 -0800'
$ date --date="@1631716521"
date: invalid date '@1631716521'
```
I am currently using the version `0.0.7` on NixOS 20.05.

Also, thanks for all your work!