M54: harden operations and demo resilience

frontend/src/i18n/brusselsDateTime.ts

@@ -21,7 +21,13 @@ function partsAt(value: Date): Record<string, string> {
 
 export function toBrusselsDateTimeLocal(value: Date): string {
   const parts = partsAt(value);
-  return `${parts.year}-${parts.month}-${parts.day}T${parts.hour}:${parts.minute}`;
+  return `${toBrusselsDate(value)}T${parts.hour}:${parts.minute}`;
+}
+
+/** Render an instant as the Fleet Ops Europe/Brussels calendar date (YYYY-MM-DD). */
+export function toBrusselsDate(value: Date): string {
+  const parts = partsAt(value);
+  return `${parts.year}-${parts.month}-${parts.day}`;
 }
 
 export function brusselsDateTimeFromNow(hours: number): string {
@@ -42,7 +48,21 @@ export function brusselsLocalToIso(value: string): string {
     const represented = Date.UTC(+parts.year, +parts.month - 1, +parts.day, +parts.hour, +parts.minute);
     candidate += wallClockUtc - represented;
   }
-  return new Date(candidate).toISOString();
+  const result = new Date(candidate);
+  // Date.UTC normalises impossible calendar values and Brussels' spring transition
+  // contains a wall-clock hour that does not exist. Never silently shift either one.
+  if (toBrusselsDateTimeLocal(result) !== value) {
+    throw new Error("Invalid or non-existent Brussels date-time");
+  }
+  return result.toISOString();
+}
+
+export function tryBrusselsLocalToIso(value: string): string | null {
+  try {
+    return brusselsLocalToIso(value);
+  } catch {
+    return null;
+  }
 }
 
 /** Start of the given Brussels calendar day (YYYY-MM-DD) as a UTC ISO instant. */