/** Date helpers for the temporal pages (UTC, YYYY-MM-DD). Server-safe, no data. */
export const ISO_DAY = /^\d{4}-\d{2}-\d{2}$/;

export function todayUtc(): string {
  return new Date().toISOString().slice(0, 10);
}
export function daysBefore(iso: string, days: number): string {
  const d = new Date(`${iso}T00:00:00Z`);
  d.setUTCDate(d.getUTCDate() - days);
  return d.toISOString().slice(0, 10);
}
export function monthsBefore(iso: string, months: number): string {
  const d = new Date(`${iso}T00:00:00Z`);
  d.setUTCMonth(d.getUTCMonth() - months);
  return d.toISOString().slice(0, 10);
}
export function firstOfMonth(iso: string): string {
  return `${iso.slice(0, 7)}-01`;
}
export function firstOfQuarter(iso: string): string {
  const m = Number(iso.slice(5, 7));
  const q = Math.floor((m - 1) / 3) * 3 + 1;
  return `${iso.slice(0, 4)}-${String(q).padStart(2, '0')}-01`;
}
export function validDay(v: string | undefined | null): string | null {
  if (!v || !ISO_DAY.test(v)) return null;
  const d = new Date(`${v}T00:00:00Z`);
  return Number.isNaN(d.getTime()) ? null : v;
}