M39: harden application and acceptance gates

frontend/src/i18n/brusselsDateTime.ts

@@ -44,3 +44,15 @@ export function brusselsLocalToIso(value: string): string {
   }
   return new Date(candidate).toISOString();
 }
+
+/** Start of the given Brussels calendar day (YYYY-MM-DD) as a UTC ISO instant. */
+export function brusselsDayStartIso(day: string): string {
+  return brusselsLocalToIso(`${day}T00:00`);
+}
+
+/** Start of the day *after* the given Brussels calendar day, i.e. an exclusive upper bound. */
+export function brusselsNextDayStartIso(day: string): string {
+  const [year, month, date] = day.split("-").map(Number);
+  const next = new Date(Date.UTC(year, month - 1, date + 1));
+  return brusselsLocalToIso(`${next.toISOString().slice(0, 10)}T00:00`);
+}