hello-world.md lacks a date

```
err: Error: `NaN` is not a valid date!
```

Initializing a new blog and running the server results in an error because hello-world.md lacks a date.

Adding a date to source/_posts/hello-world.md resolves the issue.

```
hexo: 5.4.0
hexo-cli: 4.3.0
os: freebsd 12.2-RELEASE-p2
node: 14.17.6
v8: 8.4.371.23-node.76
uv: 1.41.0
zlib: 1.2.11
brotli: 1.0.9
ares: 1.17.1
modules: 83
nghttp2: 1.43.0
napi: 8
llhttp: 2.1.3
openssl: 1.1.1l
cldr: 39.0
icu: 69.1
tz: 2021a
unicode: 13.0
```