second version

frontend/src/lib/pragueDate.ts

@@ -16,3 +16,16 @@ export function instantPragueDay(iso: string): string {
     day: '2-digit',
   }).format(new Date(iso))
 }
+
+/** Kalendářní den v Praze posunutý o N dní (od ref YYYY-MM-DD v Praze). */
+export function pragueAddCalendarDays(ymd: string, deltaDays: number): string {
+  const [y, m, d] = ymd.split('-').map(Number)
+  const utc = new Date(Date.UTC(y, m - 1, d, 12, 0, 0))
+  utc.setUTCDate(utc.getUTCDate() + deltaDays)
+  return new Intl.DateTimeFormat('en-CA', {
+    timeZone: 'Europe/Prague',
+    year: 'numeric',
+    month: '2-digit',
+    day: '2-digit',
+  }).format(utc)
+}