LetterHtml.Render already receives the letter's instant and uses it
for the letterhead date. The body's "datum" placeholder resolved
through ResolveAuto, which ignored that instant and read the wall
clock instead. This is not a shipped bug today, because every current
caller passes Now() at render time. It becomes one the moment Render
runs with a historical instant (an archive re-render, a back-dated
letter): the letterhead and the body would then disagree within one
document.
Thread the existing "at" parameter down through RenderParagraphs and
RenderNode into ResolveAuto's "datum" case. Render's own signature,
and every call site, stays unchanged.
Add two tests with a fixed historical "at": one pins the body's
rendered date to the expected Dutch string, the other asserts the
letterhead date and the body date agree. Both fail red against the
old code, showing today's date instead of the pinned one.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
ehoandClaude Opus 5
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