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>
