Results 6 comments of Zan Pan

The [`cron`](https://pkg.go.dev/github.com/robfig/cron/v3#hdr-Intervals) lib in golang has an implementation.

It is also OK to return an `ETag` header. Without either `Last-Modified` or `ETag`, it is difficult for us to ensure that the content is unchanged.

Any blockers for the stabilization of the `lazy_cell` in std?