backend/src/main/java/de/fete/adapter/in/web/GlobalExceptionHandler.java

@@ -1,5 +1,6 @@
 package de.fete.adapter.in.web;
 
+import de.fete.application.service.EventExpiredException;
 import de.fete.application.service.EventNotFoundException;
 import de.fete.application.service.ExpiryDateInPastException;
 import de.fete.application.service.InvalidTimezoneException;
@@ -59,6 +60,19 @@ public class GlobalExceptionHandler extends ResponseEntityExceptionHandler {
         .body(problemDetail);
   }
 
+  /** Handles RSVP on expired event. */
+  @ExceptionHandler(EventExpiredException.class)
+  public ResponseEntity<ProblemDetail> handleEventExpired(
+      EventExpiredException ex) {
+    ProblemDetail problemDetail = ProblemDetail.forStatusAndDetail(
+        HttpStatus.CONFLICT, ex.getMessage());
+    problemDetail.setTitle("Event Expired");
+    problemDetail.setType(URI.create("urn:problem-type:event-expired"));
+    return ResponseEntity.status(HttpStatus.CONFLICT)
+        .contentType(MediaType.APPLICATION_PROBLEM_JSON)
+        .body(problemDetail);
+  }
+
   /** Handles event not found. */
   @ExceptionHandler(EventNotFoundException.class)
   public ResponseEntity<ProblemDetail> handleEventNotFound(

backend/src/main/java/de/fete/application/service/RsvpService.java

@@ -8,6 +8,8 @@ import de.fete.domain.port.in.CountAttendeesByEventUseCase;
 import de.fete.domain.port.in.CreateRsvpUseCase;
 import de.fete.domain.port.out.EventRepository;
 import de.fete.domain.port.out.RsvpRepository;
+import java.time.Clock;
+import java.time.LocalDate;
 import org.springframework.stereotype.Service;
 
 /** Application service implementing RSVP creation. */
@@ -16,13 +18,16 @@ public class RsvpService implements CreateRsvpUseCase, CountAttendeesByEventUseC
 
   private final EventRepository eventRepository;
   private final RsvpRepository rsvpRepository;
+  private final Clock clock;
 
   /** Creates a new RsvpService. */
   public RsvpService(
       EventRepository eventRepository,
-      RsvpRepository rsvpRepository) {
+      RsvpRepository rsvpRepository,
+      Clock clock) {
     this.eventRepository = eventRepository;
     this.rsvpRepository = rsvpRepository;
+    this.clock = clock;
   }
 
   @Override
@@ -30,6 +35,10 @@ public class RsvpService implements CreateRsvpUseCase, CountAttendeesByEventUseC
     Event event = eventRepository.findByEventToken(eventToken)
         .orElseThrow(() -> new EventNotFoundException(eventToken.value()));
 
+    if (!event.getExpiryDate().isAfter(LocalDate.now(clock))) {
+      throw new EventExpiredException(eventToken.value());
+    }
+
     var rsvp = new Rsvp();
     rsvp.setRsvpToken(RsvpToken.generate());
     rsvp.setEventId(event.getId());

backend/src/test/java/de/fete/adapter/in/web/EventControllerIntegrationTest.java

@@ -349,6 +349,22 @@ class EventControllerIntegrationTest {
         .andExpect(jsonPath("$.type").value("urn:problem-type:event-not-found"));
   }
 
+  @Test
+  void createRsvpForExpiredEventReturns409() throws Exception {
+    EventJpaEntity event = seedEvent(
+        "Expired Party", null, "Europe/Berlin",
+        null, LocalDate.now().minusDays(1));
+
+    var request = new CreateRsvpRequest().name("Late Guest");
+
+    mockMvc.perform(post("/api/events/" + event.getEventToken() + "/rsvps")
+            .contentType(MediaType.APPLICATION_JSON)
+            .content(objectMapper.writeValueAsString(request)))
+        .andExpect(status().isConflict())
+        .andExpect(content().contentTypeCompatibleWith("application/problem+json"))
+        .andExpect(jsonPath("$.type").value("urn:problem-type:event-expired"));
+  }
+
   private EventJpaEntity seedEvent(
       String title, String description, String timezone,
       String location, LocalDate expiryDate) {

backend/src/test/java/de/fete/application/service/RsvpServiceTest.java

@@ -12,6 +12,8 @@ import de.fete.domain.model.OrganizerToken;
 import de.fete.domain.model.Rsvp;
 import de.fete.domain.port.out.EventRepository;
 import de.fete.domain.port.out.RsvpRepository;
+import java.time.Clock;
+import java.time.Instant;
 import java.time.LocalDate;
 import java.time.OffsetDateTime;
 import java.time.ZoneId;
@@ -28,6 +30,9 @@ import org.mockito.junit.jupiter.MockitoExtension;
 class RsvpServiceTest {
 
   private static final ZoneId ZONE = ZoneId.of("Europe/Berlin");
+  private static final Instant NOW = Instant.parse("2026-03-08T12:00:00Z");
+  private static final Clock FIXED_CLOCK = Clock.fixed(NOW, ZONE);
+  private static final LocalDate TODAY = LocalDate.ofInstant(NOW, ZONE);
 
   @Mock
   private EventRepository eventRepository;
@@ -39,7 +44,7 @@ class RsvpServiceTest {
 
   @BeforeEach
   void setUp() {
-    rsvpService = new RsvpService(eventRepository, rsvpRepository);
+    rsvpService = new RsvpService(eventRepository, rsvpRepository, FIXED_CLOCK);
   }
 
   @Test
@@ -95,6 +100,28 @@ class RsvpServiceTest {
     assertThat(result.getName()).isEqualTo("Max");
   }
 
+  @Test
+  void createRsvpThrowsWhenEventExpired() {
+    var event = buildActiveEvent();
+    event.setExpiryDate(TODAY.minusDays(1));
+    EventToken token = event.getEventToken();
+    when(eventRepository.findByEventToken(token)).thenReturn(Optional.of(event));
+
+    assertThatThrownBy(() -> rsvpService.createRsvp(token, "Late Guest"))
+        .isInstanceOf(EventExpiredException.class);
+  }
+
+  @Test
+  void createRsvpThrowsWhenEventExpiresToday() {
+    var event = buildActiveEvent();
+    event.setExpiryDate(TODAY);
+    EventToken token = event.getEventToken();
+    when(eventRepository.findByEventToken(token)).thenReturn(Optional.of(event));
+
+    assertThatThrownBy(() -> rsvpService.createRsvp(token, "Late Guest"))
+        .isInstanceOf(EventExpiredException.class);
+  }
+
   private Event buildActiveEvent() {
     var event = new Event();
     event.setId(1L);
@@ -103,7 +130,7 @@ class RsvpServiceTest {
     event.setTitle("Test Event");
     event.setDateTime(OffsetDateTime.of(2026, 6, 15, 20, 0, 0, 0, ZoneOffset.ofHours(2)));
     event.setTimezone(ZONE);
-    event.setExpiryDate(LocalDate.of(2026, 7, 15));
+    event.setExpiryDate(TODAY.plusDays(30));
     event.setCreatedAt(OffsetDateTime.now());
     return event;
   }