oneshot

lib/rsh26pool/jst.ex

@@ -0,0 +1,52 @@
+defmodule Rsh26pool.JST do
+  @moduledoc """
+  Helpers for converting between UTC (how timestamps are stored) and
+  Japan Standard Time (JST, UTC+9) which is what the UI shows and accepts.
+
+  A full time zone database is not configured for this demo, and JST has no
+  daylight saving time, so a fixed +9h offset is sufficient and dependency free.
+  """
+
+  @offset_seconds 9 * 3600
+
+  @doc """
+  Parses a `datetime-local` form value (e.g. `"2026-07-04T15:30"`), interpreting
+  it as wall-clock JST, and returns the corresponding UTC `DateTime`.
+  """
+  @spec parse_local(String.t()) :: {:ok, DateTime.t()} | :error
+  def parse_local(value) when is_binary(value) do
+    with {:ok, naive} <- NaiveDateTime.from_iso8601(ensure_seconds(value)) do
+      utc =
+        naive
+        |> DateTime.from_naive!("Etc/UTC")
+        |> DateTime.add(-@offset_seconds, :second)
+        |> DateTime.truncate(:second)
+
+      {:ok, utc}
+    else
+      _ -> :error
+    end
+  end
+
+  def parse_local(_), do: :error
+
+  @doc "Converts a stored UTC `DateTime` to a JST `DateTime` for display."
+  @spec to_local(DateTime.t()) :: DateTime.t()
+  def to_local(%DateTime{} = dt), do: DateTime.add(dt, @offset_seconds, :second)
+
+  @doc "Formats a stored UTC `DateTime` as a human friendly JST string."
+  @spec format(DateTime.t() | nil) :: String.t()
+  def format(nil), do: ""
+
+  def format(%DateTime{} = dt) do
+    l = to_local(dt)
+
+    "~4..0B/~2..0B/~2..0B ~2..0B:~2..0B"
+    |> :io_lib.format([l.year, l.month, l.day, l.hour, l.minute])
+    |> to_string()
+  end
+
+  defp ensure_seconds(value) do
+    if Regex.match?(~r/T\d{2}:\d{2}$/, value), do: value <> ":00", else: value
+  end
+end