What date formatting does a Gleam HTTP response header need?

birl provides format_date, which you could call for this:

 import gleam/http/elli
 import gleam/http/response.{type Response}
 import gleam/http/request.{type Request}
 import gleam/bytes_builder.{type BytesBuilder}
 import birl
 import time.{Iso8601}

 // Set up an HTTP service
 //
 pub fn serve_incoming(_incoming: Request(t)) -> Response(BytesBuilder) {
   let payload = bytes_builder.from_string("Hello there!")

   response.new(200)
   |> response.prepend_header("content-type", "text/plain")
   |> response.prepend_header("server", "gleam")
   |> response.prepend_header("date", http_date(birl.now()))
   |> response.set_body(payload)
 }

 fn http_date(moment: Iso8601) -> String {
   let formatted = birl.format_date(moment, "EEE, dd MMM yyyy HH:mm:ss 'GMT'")
   formatted
 }

 pub fn main() {
   elli.become(serve_incoming, on_port: 8181)
 }

Apparently birl includes a function named to_http (https://hexdocs.pm/birl/birl.html#to_http).

 birl.to_http(birl.now())

There's now a function in the official gleam_time package for this:

https://gleam-time.hexdocs.pm/gleam/time/timestamp.html#to_http_date

 timestamp.from_unix_seconds(1)
 |> timestamp.to_http_date
 // -> "Thu, 01 Jan 1970 00:00:01 GMT"