diff --git a/internal/observabilityproxy/logsparams.go b/internal/observabilityproxy/logsparams.go
index 220a06762..ec6043c6f 100644
--- a/internal/observabilityproxy/logsparams.go
+++ b/internal/observabilityproxy/logsparams.go
@@ -160,10 +160,11 @@ func window(in url.Values, now time.Time, lim LogsLimits) (time.Time, time.Time,
 	if end.Sub(start) > lim.MaxRange {
 		start = end.Add(-lim.MaxRange)
 	}
-	for k, t := range map[string]time.Time{"start": start, "end": end} {
-		if herr := sendableTime(k, t); herr != nil {
-			return time.Time{}, time.Time{}, herr
-		}
+	if herr := sendableTime("end", end); herr != nil {
+		return time.Time{}, time.Time{}, herr
+	}
+	if herr := sendableTime("start", start); herr != nil {
+		return time.Time{}, time.Time{}, herr
 	}
 	return start.UTC(), end.UTC(), nil
 }
@@ -337,7 +338,7 @@ func scaleDuration(f float64, unit time.Duration) (time.Duration, bool) {
 // actually sent are checked; an older start is clamped first.
 func sendableTime(key string, t time.Time) *HTTPError {
 	if t.Before(time.Unix(0, math.MinInt64)) || t.After(time.Unix(0, math.MaxInt64)) {
-		return errorf(http.StatusBadRequest, "invalid %s: %s is outside 1677..2262", key, t.Format(time.RFC3339))
+		return errorf(http.StatusBadRequest, "invalid %s: %s is outside 1677..2262", key, t.UTC().Format(time.RFC3339Nano))
 	}
 	return nil
 }
diff --git a/internal/observabilityproxy/logsparams_test.go b/internal/observabilityproxy/logsparams_test.go
index f7d21753a..82ed29190 100644
--- a/internal/observabilityproxy/logsparams_test.go
+++ b/internal/observabilityproxy/logsparams_test.go
@@ -17,6 +17,7 @@ limitations under the License.
 package observabilityproxy
 
 import (
+	"math"
 	"net/http"
 	"net/url"
 	"sort"
@@ -408,3 +409,26 @@ func TestBuildLogsParamsOldTimesClamped(t *testing.T) {
 		t.Errorf("start_offset = %s, want the %s cap", got, lim.MaxTailResume)
 	}
 }
+
+// TestBuildLogsParamsSendableBoundary pins the inclusive int64-nanosecond
+// bound and that the refusal names the instant exactly.
+func TestBuildLogsParamsSendableBoundary(t *testing.T) {
+	last := time.Unix(0, math.MaxInt64).UTC()
+	in := url.Values{"start": {last.Add(-time.Hour).Format(time.RFC3339Nano)}, "end": {last.Format(time.RFC3339Nano)}}
+	if _, herr := BuildLogsParams("query", in, "tenant-a", testNow, testLimits()); herr != nil {
+		t.Fatalf("the last int64 nanosecond refused: %v", herr)
+	}
+	past := last.Add(time.Nanosecond).Format(time.RFC3339Nano)
+	in = url.Values{"start": {last.Add(-time.Hour).Format(time.RFC3339Nano)}, "end": {past}}
+	_, herr := BuildLogsParams("query", in, "tenant-a", testNow, testLimits())
+	if herr == nil || !strings.Contains(herr.Message, "invalid end") || !strings.Contains(herr.Message, past) {
+		t.Fatalf("one nanosecond past answered %v, want end refused naming %s", herr, past)
+	}
+	// Both out of range: the answer names end, deterministically.
+	for range 20 {
+		_, herr := BuildLogsParams("query", url.Values{"start": {"0001-01-01T00:00:00Z"}, "end": {"0001-01-02T00:00:00Z"}}, "tenant-a", testNow, testLimits())
+		if herr == nil || !strings.Contains(herr.Message, "invalid end") {
+			t.Fatalf("answer = %v, want end named", herr)
+		}
+	}
+}