Keep school dates in the school's timezone

The container runs on UTC and the school does not: at 00:30 in Erfurt the
process clock still says yesterday, so anything deriving "today" from a Date
would prepare the wrong school day twice a night.

core/dates.ts holds calendar dates as plain YYYY-MM-DD strings, which is what
a school day is — today in Europe/Berlin, whole-day arithmetic anchored at
noon UTC so no daylight-saving change can shift a date, the German weekday and
day formats, and the compact form the timetable API takes.

isCalendarDate round-trips rather than only matching a shape: Date.parse turns
2026-02-30 into March 2nd instead of refusing it, so a shape check alone would
let a caller read a different day than it asked for.

germanDate moves here from mcp/prompts.ts, its only previous home, so the
timezone is stated in one place.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/dates.test.ts

@@ -0,0 +1,66 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import {
+	addDays,
+	compactDate,
+	daysBetween,
+	germanDate,
+	germanDay,
+	germanWeekday,
+	isCalendarDate,
+	schoolToday,
+} from '../src/core/dates.ts';
+
+describe('schoolToday', () => {
+	it('is the school\'s date, not the container\'s', () => {
+		// 23:30 UTC in January is already tomorrow in Berlin (UTC+1) — the case
+		// that would make a nightly briefing prepare the wrong day.
+		assert.equal(schoolToday(new Date('2026-01-15T23:30:00Z')), '2026-01-16');
+		// And in summer the offset is two hours.
+		assert.equal(schoolToday(new Date('2026-07-15T22:30:00Z')), '2026-07-16');
+		assert.equal(schoolToday(new Date('2026-07-15T21:30:00Z')), '2026-07-15');
+	});
+});
+
+describe('addDays and daysBetween', () => {
+	it('walks calendar dates across a daylight-saving change', () => {
+		// Clocks go forward in the EU on 2026-03-29, so a day of 23 hours lies
+		// inside this range.
+		assert.equal(addDays('2026-03-28', 2), '2026-03-30');
+		assert.equal(daysBetween('2026-03-28', '2026-03-30'), 2);
+		// And back again in October.
+		assert.equal(addDays('2026-10-24', 2), '2026-10-26');
+	});
+
+	it('crosses months and years', () => {
+		assert.equal(addDays('2026-12-31', 1), '2027-01-01');
+		assert.equal(addDays('2026-01-01', -1), '2025-12-31');
+		assert.equal(addDays('2028-02-28', 1), '2028-02-29');
+		assert.equal(daysBetween('2026-09-21', '2026-09-18'), -3);
+	});
+
+	it('refuses something that is not a calendar date', () => {
+		assert.throws(() => addDays('21.09.2026', 1), /YYYY-MM-DD/);
+	});
+});
+
+describe('calendar date helpers', () => {
+	it('recognises real dates only', () => {
+		assert.equal(isCalendarDate('2026-09-21'), true);
+		assert.equal(isCalendarDate('2026-13-01'), false);
+		assert.equal(isCalendarDate('2026-02-30'), false);
+		assert.equal(isCalendarDate('2026-9-21'), false);
+		assert.equal(isCalendarDate('heute'), false);
+	});
+
+	it('formats the way a German timetable reads', () => {
+		assert.equal(germanWeekday('2026-09-21'), 'Montag');
+		assert.equal(germanWeekday('2026-09-17'), 'Donnerstag');
+		assert.equal(germanDay('2026-09-21'), '21.09.2026');
+		assert.equal(compactDate('2026-09-21'), 20_260_921);
+	});
+
+	it('gives a moment the school\'s day, with its weekday', () => {
+		assert.equal(germanDate(new Date('2026-09-14T23:30:00Z')), 'Dienstag, 15.09.2026');
+	});
+});

test/prompts.test.ts

@@ -1,7 +1,8 @@
 import assert from 'node:assert/strict';
 import { describe, it } from 'node:test';
 import { ErrorCode } from '@modelcontextprotocol/sdk/types.js';
-import { argumentText, examPrompt, germanDate, resolveTarget, summaryPrompt, type Target } from '../src/mcp/prompts.ts';
+import { germanDate } from '../src/core/dates.ts';
+import { argumentText, examPrompt, resolveTarget, summaryPrompt, type Target } from '../src/mcp/prompts.ts';
 import { ProtocolError } from '../src/mcp/tools/result.ts';
 
 const CANDIDATES: Target[] = [
@@ -151,3 +152,4 @@ describe('germanDate', () => {
 		assert.equal(germanDate(new Date('2026-09-14T23:30:00Z')), 'Dienstag, 15.09.2026');
 	});
 });
+