diff --git a/internal/restapi/schedule_for_stop_handler.go b/internal/restapi/schedule_for_stop_handler.go
index 8b33e2d74..adbabb122 100644
--- a/internal/restapi/schedule_for_stop_handler.go
+++ b/internal/restapi/schedule_for_stop_handler.go
@@ -29,6 +29,16 @@ func (api *RestAPI) scheduleForStopHandler(w http.ResponseWriter, r *http.Reques
 	// Get the date parameter or use current date
 	dateParam := r.URL.Query().Get("date")
 
+	// An unparseable date is a field error even when the ID resolves to nothing, so it has
+	// to be caught before the agency lookup below. Resolving the date to a service date
+	// needs that agency's timezone, so the parse itself stays where it is.
+	if dateParam != "" {
+		if err := utils.ValidateServiceDate(dateParam); err != nil {
+			api.validationErrorResponse(w, r, map[string][]string{"date": {err.Error()}})
+			return
+		}
+	}
+
 	agency, err := api.GtfsManager.GtfsDB.Queries.GetAgency(ctx, agencyID)
 	if err != nil {
 		api.sendNotFound(w, r)
@@ -46,12 +56,10 @@ func (api *RestAPI) scheduleForStopHandler(w http.ResponseWriter, r *http.Reques
 
 	if dateParam != "" {
 		var err error
+		// The format was validated above, so this only fails on an unusable agency timezone.
 		startOfDay, err = utils.ParseDate(dateParam, loc)
 		if err != nil {
-			fieldErrors := map[string][]string{
-				"date": {err.Error()},
-			}
-			api.validationErrorResponse(w, r, fieldErrors)
+			api.serverErrorResponse(w, r, err)
 			return
 		}
 
diff --git a/internal/restapi/schedule_for_stop_handler_test.go b/internal/restapi/schedule_for_stop_handler_test.go
index fa466e07e..382365fb4 100644
--- a/internal/restapi/schedule_for_stop_handler_test.go
+++ b/internal/restapi/schedule_for_stop_handler_test.go
@@ -347,6 +347,68 @@ func TestScheduleForStopHandlerInvalidDateFormat(t *testing.T) {
 	}
 }
 
+func TestScheduleForStopHandlerDateValidationPrecedesLookup(t *testing.T) {
+	api := createTestApi(t)
+	defer api.Shutdown()
+
+	knownStopID := utils.FormCombinedID(mustGetAgencies(t, api)[0].ID, mustGetStop(t, api).ID)
+
+	tests := []struct {
+		name             string
+		stopID           string
+		date             string
+		expectedStatus   int
+		expectFieldError bool
+	}{
+		{
+			name:             "unknown agency with an invalid date",
+			stopID:           "99_1001",
+			date:             "garbage",
+			expectedStatus:   http.StatusBadRequest,
+			expectFieldError: true,
+		},
+		{
+			name:             "known agency with an invalid date",
+			stopID:           knownStopID,
+			date:             "garbage",
+			expectedStatus:   http.StatusBadRequest,
+			expectFieldError: true,
+		},
+		{
+			name:           "unknown agency with a valid date",
+			stopID:         "99_1001",
+			date:           "2025-06-12",
+			expectedStatus: http.StatusNotFound,
+		},
+		{
+			name:           "unknown stop with a valid date",
+			stopID:         "25_9999999",
+			date:           "2025-06-12",
+			expectedStatus: http.StatusNotFound,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			endpoint := "/api/where/schedule-for-stop/" + tt.stopID + ".json?key=org.onebusaway.iphone&date=" + tt.date
+			resp, model := serveApiAndRetrieveEndpoint(t, api, endpoint)
+
+			assert.Equal(t, tt.expectedStatus, resp.StatusCode)
+			assert.Equal(t, tt.expectedStatus, model.Code)
+
+			if !tt.expectFieldError {
+				return
+			}
+
+			data, ok := model.Data.(map[string]any)
+			require.True(t, ok)
+			fieldErrors, ok := data["fieldErrors"].(map[string]any)
+			require.True(t, ok)
+			assert.NotEmpty(t, fieldErrors["date"])
+		})
+	}
+}
+
 func TestScheduleForStopHandlerScheduleContent(t *testing.T) {
 	api := createTestApi(t)
 	defer api.Shutdown()
diff --git a/internal/utils/validation.go b/internal/utils/validation.go
index 34c9142c0..34bfdbe86 100644
--- a/internal/utils/validation.go
+++ b/internal/utils/validation.go
@@ -107,6 +107,14 @@ func ValidateDate(date string) error {
 	return nil
 }
 
+// ValidateServiceDate reports whether a service date parameter is parseable, in either
+// of the forms ParseDate accepts. Handlers use it to reject a malformed date before
+// looking up the agency whose timezone ParseDate then resolves the date against.
+func ValidateServiceDate(date string) error {
+	_, err := ParseDate(date, time.UTC)
+	return err
+}
+
 // SanitizeInput removes HTML tags and other potentially dangerous content
 func SanitizeInput(input string) string {
 	// Remove HTML tags