diff --git a/internal/web/service/calendar_renew.go b/internal/web/service/calendar_renew.go
index aa5b47786..e5856151a 100644
--- a/internal/web/service/calendar_renew.go
+++ b/internal/web/service/calendar_renew.go
@@ -93,6 +93,10 @@ func localCalendarDateStart(date time.Time, loc *time.Location) (time.Time, bool
 	return start, localDate(start).Equal(date)
 }
 
+// maxClientResetDays bounds an interval renewal at a century: a larger one, which a node can
+// report, wraps int64 once turned into milliseconds.
+const maxClientResetDays = 36500
+
 func nextClientRenewal(expiry int64, reset, day, weekday int, loc *time.Location) int64 {
 	if day > 0 {
 		return nextCalendarRenewal(time.UnixMilli(expiry), day, loc).UnixMilli()
@@ -100,6 +104,9 @@ func nextClientRenewal(expiry int64, reset, day, weekday int, loc *time.Location
 	if weekday > 0 {
 		return nextWeeklyRenewal(time.UnixMilli(expiry), weekday, loc).UnixMilli()
 	}
+	if reset > maxClientResetDays {
+		return expiry
+	}
 	return expiry + int64(reset)*86400000
 }
 
diff --git a/internal/web/service/calendar_weekly_bound_test.go b/internal/web/service/calendar_weekly_bound_test.go
index 00d992646..6c90524e4 100644
--- a/internal/web/service/calendar_weekly_bound_test.go
+++ b/internal/web/service/calendar_weekly_bound_test.go
@@ -76,3 +76,21 @@ func TestWeeklyRenewalSearchFailsClosed(t *testing.T) {
 		})
 	}
 }
+
+// A node reports the renewal interval unchecked; one that wraps int64 when turned into
+// milliseconds made the catch-up loop step through time for seconds per client.
+func TestCatchUpClientRenewalRefusesAnOversizedInterval(t *testing.T) {
+	now := time.Now().UnixMilli()
+	for _, reset := range []int{106751991167, maxClientResetDays + 1} {
+		traffic := &xray.ClientTraffic{Email: "c", Reset: reset, ExpiryTime: now - 86400000}
+		start := time.Now()
+		expiry, renewals := catchUpClientRenewal(traffic, now, time.UTC)
+		if expiry != traffic.ExpiryTime || renewals != 0 || time.Since(start) > 100*time.Millisecond {
+			t.Fatalf("reset %d: expiry %d, renewals %d after %v; want no renewal", reset, expiry, renewals, time.Since(start))
+		}
+	}
+	traffic := &xray.ClientTraffic{Email: "c", Reset: maxClientResetDays, ExpiryTime: now - 86400000}
+	if _, renewals := catchUpClientRenewal(traffic, now, time.UTC); renewals != 1 {
+		t.Fatalf("a %d-day interval renewed %d times, want once", maxClientResetDays, renewals)
+	}
+}
diff --git a/internal/web/service/client_crud.go b/internal/web/service/client_crud.go
index 773f0d869..c1bac93dd 100644
--- a/internal/web/service/client_crud.go
+++ b/internal/web/service/client_crud.go
@@ -86,6 +86,9 @@ func validateClientRenewal(client model.Client) error {
 	if err := validateClientResetDay(client.ResetDay); err != nil {
 		return err
 	}
+	if client.Reset > maxClientResetDays {
+		return common.NewError("client reset must not exceed", maxClientResetDays, "days")
+	}
 	if client.ResetWeekday < 0 || client.ResetWeekday > 7 {
 		return common.NewError("client resetWeekday must be between 0 and 7, got:", client.ResetWeekday)
 	}
diff --git a/internal/web/service/client_renewal_write_validation_test.go b/internal/web/service/client_renewal_write_validation_test.go
index b76d9f137..ed950f500 100644
--- a/internal/web/service/client_renewal_write_validation_test.go
+++ b/internal/web/service/client_renewal_write_validation_test.go
@@ -20,6 +20,7 @@ func TestClientRenewalWriteValidation(t *testing.T) {
 			{"weekly and interval", 7, 0, 3, "client weekly renewal cannot be combined with reset or resetDay"},
 			{"weekly and monthly", 0, 1, 3, "client weekly renewal cannot be combined with reset or resetDay"},
 			{"invalid weekday", 0, 0, 8, "client resetWeekday must be between 0 and 7, got: 8"},
+			{"oversized interval", maxClientResetDays + 1, 0, 0, "client reset must not exceed 36500 days"},
 		} {
 			t.Run(operation+"/"+schedule.name, func(t *testing.T) {
 				setupConflictDB(t)